WMG / Star Fox 64

Sector Z is retribution for all the times you've stolen your wingmen's kills.

"Hey! He was mine!" —Slippy
"What's the big idea, Fox?" —Peppy
"Go find your own target, Fox!" —Falco

Fox's wingmen, fed up with Fox's greedy tendencies to kill their targets just so he can get medals and high scores, take advantage of Sector Z to secretly oppose Fox by shooting down the 11-point missiles and deprive him of the chance to satiate his scoring hunger. And Kat's repeat appearance, should the team come from Zoness? She didn't return of her own accord; Fox's wingmen paid her to assist in teaching Fox a lesson.

Alternatively, they are unaware of the points system

When their kills are stolen by Fox, they simply get upset because they really wanted to kill that particular enemy, only for someone else (Fox) to butt in and take that away from them. Meanwhile, in Sector Z, they don't know that Fox wants to destroy everything by himself or don't believe he can do it, so to avoid unnecessary risks they try to pitch in — if they didn't and let Fox try to handle it, it would be very possible that he'd mess up and cause Great Fox to get damaged, which wouldn't look good on him and could damage his reputation for reckless behavior.

"Barrel Roll" is the correct term

It may be more accurately known as an aileron roll to us, but in the Lylat system, they call it a barrel roll because that's the term they use; it's correct to them. It may also be slang due to "barrel" having a better ring to it than "aileron".

When you change course, you change the general outcome

For instance if you defeat the alternate boss in Corneria and choose to go to Meteo instead of Sector Y, the actual outcome is identical to you having fought Granga instead, meaning that in-universe Star Fox never encountered the Attack Carrier. This would also explain other, contradicting cases, like Fox reacting to Bill's presence in Sector X as if he had died even if the base on Katina was successfully defended or Slippy inexplicably winding up on Titania even if you defeat Spyborg before he can slap him away — by choosing to change course, you're opting for the other outcome which means that, in actuality, Star Fox failed the mission at hand. The one exception to this is Sector Z, where Great Fox doesn't suddenly sustain damage if the course is changed; this particular instance is Star Fox having a choice of where to strike rather than altering the outcome.