Dragon's breath rounds for shotguns against a single man, even someone like John Wick, may seem like overkill, but considering how prevalent body armor is in the assassin world, setting someone aflame would be an effective countermeasure.
Mr. Nobody using a modernized lever-action with a barrel threaded for a suppressor just seems like Rule of CoolSchizo Tech to fit in with his cowboy motif, but it actually makes practical sense: Manually operated firearms like bolt-actions and lever-actions make far less sound when suppressed due to the absence of mechanical noise from the shot, and a lever-action is much better suited to close-range fighting than a bolt-action would be.
Also, legally speaking, lever action rifles (and boltguns) often have less restrictions than semi-autos or assault rifles, around the world. Nobody doesn't really need to worry about legality, but it couldn't hurt.
And if you need to squeeze every last bit of accuracy out of a weapon, a manual action is better than semi-automatic or automatic. You only have to compensate for the recoil, not the movement of the action cycling. Mr. Nobody likes playing Cold Sniper, so a manual action would be preferable to make his shots as accurate as possible.
It is briefly mentioned "The Tarasovs" are seeking an audience with the High Table (presumably to discuss the John Wick situation). This faction is presumably headed by Abram Tarasov, Viggo's brother whom we last saw in Chapter 2 making peace with Wick (and never even considering breaking that peace afterwards). With that in mind, the Tarasovs are probably trying to convince the High Table to follow Abram's lead and just leave Wick alone.
When negotiating the terms of their duel, the Marquis requests blades as the weapon of choice. Considering how happy he was to stab Mr. Nobody, and the number of fencers at his estate, it suggests a skill. In hindsight once he immediately announces Caine as his stand-in it makes more sense: the Marquis knows Caine is undisputed best with a sword and Wick's gun skills would be useless.
Given the sheer numbers and resources at their disposal, the Marquis, and by extension the High Table, making the decision to bring in Caine, a retired assassin, might seem odd. However, quite simply, they were desperate. John had already ripped his way through hundreds of their men, and the High Table no longer had faith that any of their existing resources could stop him, so they had to bring in Caine. Not only was John tearing them apart physically, he'd utterly broken their belief in their own power.
Why does the Osaka Continental use bows and swords, in contrast to the rest of the assassin world? Whilst it could just be chalked up to Rule of Cool (and it certainly is that as well), there are a few pretty good reasons why they might use more traditional methods.
Guns in Japan are notoriously hard to come by, heavily controlled and policed. It's entirely possible that most assassins and related High Table organisations that operate there just can't get them reliably, and the use of them is under such greater scrutiny that they can't use them on a regular basis as their weapon of choice. Thus, there's little point in training and becoming an expert with firearms when you so seldom are able to use them. Swords and bows, however, are much easier to acquire and use.
Bulletproof suits have become standard issue amongst most of the High Table underworld, with the Elite Mooks of the High Table being Nigh-Invulnerable outside a few spots that are hard to properly exploit for anyone without John Wick's training and skill. However, a key weakness of such bulletproof material, demonstrated in previous films by John himself, is that they're not at all resistant to bladed weapons and piercing projectiles. Swords and arrows could, therefore, be part of the assassin world reacting to the introduction of the bullet proof suits, by turning to methods that will more reliably get past the protection they offer.
The Osaka Continental is also run by Shimazu, a character who's defined by his old-fashioned sense of honor and obligation, which could also account for the similarly traditionalist choice of using swords and bows.
Finally, bows are significantly quieter than even sound-suppressed firearms. Good option for assassins who work at shorter ranges.
The final duel is actually on very even ground, and not just literally. Caine is a remarkably good shot for a blind man, but in the duel, he knows exactly where John is, exactly how far away he is, and exactly what he's doing, down to which hand John uses.
The High Table is compared to the Hydra that is slain by Heracles, except that John Wick lacks his version of the cauterizing flame to deal with the replacement heads. It comes in the form of the duel. At the end of this story, just like Heracles, he dies after completing his labor (the duel) at sunrise, and becomes a deity or martyr among the criminal underworld. On that note, what is Heracles' final labor? Bringing Cerberus, the three-headed dog.
At this point in the story, it understandably stretches the limits of disbelief that the average assassin would still be dumb enough to go after John Wick instead of getting out of his way, even after seeing him go through ten people before him like they were leaves. If you re-watch the films, there are actually multiple reasons to do so aside from Rule of Cool.
The ridiculously high bounty. Going from $7-8 million in 2, $14-15 million in 3 and $40 million in 4. The one-in-a-million assassin who does succeed is set for life, however long that lasts. They could also be planning on buying their way out like Nobody the Tracker.
Being ordered to do so on the pain of death of loved ones like Caine. Another interpretation is that they are hired men that have to follow orders instead of the more ad-hoc based ones, meaning that they don't exactly have a choice.
John Wick is still a man. Every assassin expected the earlier ones that got killed to have softened him up in some form. One lucky shot is all it takes.
The bounty system is still a glorified way for the High Table to give out orders. The news of what he did meant that saying "no" may not even be an option.
The very fact that he appoints Caine to fight the duel for him (in addition to indirectly sending the entire assassin element of Paris after John) would make him seem like a Dirty Coward who's too afraid to face John Wick himself and relies on someone else to do it, which would definitely tarnish his reputation with fellow High Table members. This even reflects badly on the High Table itself: if this coward is someone that the High Table appointed to be the one to take down John Wick, then why should anyone fight for them anymore?
If Caine wins, the Marquis rids himself of both John and Winston, but he'd be forced to let Caine go free according to his deal with him. By that time, the damage is already done: John Wick and his fallen allies became martyrs as their struggle against the High Table has shown the entire criminal underworld that they aren't invincible, and there are certainly those who have suffered and will continue to suffer from the High Table's draconian rule over them like what John and his allies went through. Another uprising is very much likely, and this time Caine won't be there to fight for the Marquis again (not to mention that he already lost Chidi - his trusted right-hand man and the only henchman he could reasonably rely on).
If John wins traditionally and Caine dies, the duel is concluded with the Marquis remaining alive, but John is now a free man and Winston gets his hotel back, showing the entire underworld that there are people who fought the seemingly almighty High Table and beat them at their own game. The Marquis can't do anything to both John and Winston, too, as they won fair and square, and he must honor the agreement he made with them, so if he tries any underhanded tactics to get either John or Winston killed after this, everyone will lose any little respect they have left for him - if the Harbinger doesn't punish him accordingly first.
And lastly, just as happened in the film, the Marquis' fatal hubris allows John to put a bullet into his head when the Marquis foolishly tries to be the one to kill John Wick and makes himself part of the duel. Again, John becomes a martyr and shows the world that this man fought the High Table and won.
In any case, the Harbinger is entirely right in his assessment that the Marquis's misguided action of agreeing to a duel with John will only lead to bad things for the High Table down the line. No matter what happens, the Marquis had already lost, it's just a matter of by how much.
Even refusing the duel is likely a bad idea. If the Marquis might face tarnish to his reputation by nominating a Champion instead of facing Wick personally, how much more would he lose by declining the fight entirely? He'd likely either be seen as an utter coward who only respects the rules when they benefit him, or a bloodthirsty butcher declining an avenue to resolve the conflict with little bloodshed in favor of continuing an all-out underworld war that's already claimed two whole Continentals and hundreds of professionals. And even that all-out war was pre-existing mess the Marquis had been handed and told to resolve. The Marquis was screwed before the movie even started.
The protocol of High Table duels is believed to be a myth by most assassins in the modern day, which makes sense considering a) members of the High Table logically wouldn't want to advertise a legitimate way they can be challenged for their positions, and b) given the degree of chance in determining the conditions of the duel and the Marquis's underhanded tactics in trying to stop John Wick from even reaching the dueling ground in the first place, it's likely been an extremely long time since anyone has even tried, let alone succeeded in a duel.
Another example of the Marquis's short-sightedness in accepting the duel with John Wick is the fact that both of them have to decide the rules of engagement before the duel with the Harbinger as the arbitrator first (of which the Marquis would have the knowledge of), and there's a possibility that he and John would have to duel right then and there rather than at sunrise if John had the upper hand when deciding the time of the duel. Even if the Marquis had assigned Caine to be his champion just as he would originally, John would fight that duel in a much better shape and much more likely to win it and survives to kill the Marquis later after the duel is over.
After Winston seemingly interrupts the duel to claim that John still hadn't shot yet it wasn't just to dramatically give John a chance, it was a reminder of the strict rules of said duel which is why the Marquis stops like deer caught in the headlights— it's a brutal Morton's Fork for the Marquis: he must let John fire his gun. The Marquis could shoot John dead but that would be breaking the rules which, as it's shown throughout the movies, is an exceptionally bad idea exacerbated by the fact this is a honor duel.
The Bowery King healed John of his injuries from the previous three movies along with providing him the guns needed.
Winston taught him about the High Table duels.
Shimazu gave John Wick shelter after he killed the Elder.
The Ruska Roma restored John's place in the family and allowed him to duel with the Marquis.
Nobody the Tracker helped him deal with Chidi.
And finally, Caine helped him fully get back up the staircase after he got kicked back down to the bottom while John himself only nearly reached the top.
The Marquis having a cadre of exceptionally large assassins as his personal enforcers is not just a matter of showing off. Those enforcers are also his personal bodyguards and the Marquis is an exceptionally tall man himself. In order to provide adequate cover, his bodyguards can't all be a head shorter than the principal they are protecting.
When deciding the place for the duel, John proposes the Sacré-Cœur, which as a church is connected to the High Table's pseudo-ecclesiastical traditions, the movie's theme of preparing for one's death (and funeral), and to important moments in John's life, both on-screen (e.g., meetings with the Ruska Roma and Caine in this movie) and probably off-screen (his marriage to Helen and her funeral). The Marquis' choice, the Centre Pompidou cultural complex, is instead a "high-tech" building commissioned by and named after the French president who made efforts to modernise Paris. This fits into how the Marquis sees himself not only as a patron of culture, but as a bold and visionary leader willing to break with tradition.
The French cover of "Paint It Black" ("Marie Douceur - Marie Colère") that plays as John begins his march towards the Sacré-Cœur is appropriate on at least two levels. The original English lyrics of the song are about the grief and hopelessness of someone who has lost their love, which describes John's basic situation after the death of Helen. The French lyrics, meanwhile, are a warning from a woman to her philandering paramour that he needs to change his ways, ending with her declaring that the patient "sweet Marie" ("Marie douceur") has become "only a memory" and that he will now have to face "Marie enraged" ("Marie colère"). This mirrors John's two personae as a loving husband (what he wants to be remembered as) and as a wrathful avenger (which the death of his dog in the first movie brought back). In addition, the song accompanies the mood shift from John's heartfelt discussion with Winston and the Bowery King into the following brutal action scene, where his wrathful persona takes the reins again.
