Greta Garbo's famous quote, echoed by Lisle, isn't actually hers. It's from the White-Dwarf Starlet role Garbo played in Grand Hotel, Elizaveta Grushinskaya. Of course Lisle would use that quote to manipulate the White-Dwarf Starlet Madeline into signing up. Additionally, Elizaveta falls in love with the handsome Baron Felix von Gaigern, who has a dance-only counterpart in the stage musical referred to as the Gigolo.
Ernest becomes a great surgeon again because his right hand was affected by the serum, and he quit drinking.
Other viewers have pointed out that Liesel's pool seems to have a pink hue when Ernest lands in it. Ernest dropped the vial of potion onto the glass roof of the pool moments before he crashed through it; the potion may have diffused in the water, and seeped into Ernest's open cuts — hence why he survives a fall which otherwise would have killed him!
At Ernest's funeral, we learn that not only did he gave up drinking, remarry, and father six children (while adopting many more), but he went on to found a marriage counseling center, a center for the study of women, and an Alcoholics Anonymous chapter. Ernest didn't just fix everything wrong in own life, he went out of his way to help others who were suffering from the same things he once did.
Not only that, but the "Center for the Study of Women" suggests that he was trying to help women understand themselves (and other women) better, so they could avoid falling into the same traps as Madeline and Helen.
By trying to achieve eternal youth, Madeline and Helen's mistakes have accidentally consigned them to a much worse form of aging: decay. Their vain intentions just gave them a worse version of the path they were trying to avoid!
Ernest's skill as both a plastic surgeon and mortician makes him more than worth his weight in gold to glamorous zombies. He took the damage Helen and Madeline did to each other and made them fantastic.
Ernest refuses to take the potion even when he's hanging over an immense drop — not just because the alternative would be serving Madeline and Helen forever, but because they've reminded him of what he's seen first hand when you 'die' after drinking it. He knows the fall will 'kill' him (that he goes on to survive it without even minor injuries is frankly a miracle) and he definitely doesn't want to be stuck in a decaying body for eternity.
On that note, Ernest initially thinks Madeline and Helen 'surviving' the injuries that kill them is a miracle, before he learns the truth about the potion. Come the climax, he refuses to take the potion, but miraculously survives his fall.
Ernest escapes the party by stealing a car from someone who looks a lot like James Dean. The implication is that Dean took the potion and faked his death. This makes more sense when you remember that Dean made an appearance on a television show shortly before his death and was asked to give some advice to the viewers. He replied "Drive safely. The life you save might be mine." A subtle hint, maybe?
The fact that Madeline first hears about the Lisle's secret society at what looks to be an anti-aging clinic makes a lot of sense; it provides a plausible excuse for why her (new) clients look so young, plus it attracts potential customers in the form of aging rich people like Madeline.
Consider all of the ancient Egyptians who took the potion, they created it after all. Now consider all of the tales of shambling, groaning, moaning, angry mummies going about and killing people/or whatever and what their most likely inspiration in the movie's universe. Now we have an idea of what will be Mad and Hel's fate.
If Madeline hadn't gotten Ernst angry enough and gotten close enough to the stairs for him to push her... the neck injury wouldn't have happened. But Helen and Ernest were planning to murder her anyway, by staging an apparently alcohol fueled car crash. Since we know that by this point Madeline can't be killed... she'd have woken up charred to a crisp. Possible nightmare retardant if you consider that Ernest would likely have backed out once he had time to think it through... and even if he did go through with it the narconol might not have even effected Madeline.
Indeed, one of the original ideas for the end of the film involved Madeline and Helen chasing after Ernest in a car and crashing in a fiery explosion — and emerging from the wreckage as blackened, charred husks.
The narconal probably would have affected Madeline; it's just a tranquilizer, so there's no reason to assume it would affect her any differently from any other drug or narcotic.
Frankly, we never see anything to suggest that anyone affected by the potion isn't still basically human in every way other than the fact that they don't age and can't die of natural causes any more, so it seems perfectly reasonable to assume that they could be drugged or get drunk (at least until their bodies decayed enough after 'death' to make that impractical).
To be honest, it does look that the potion makes the living life forever, and keep their youthful appearance. But once you are dead (due to violence or accident) you're stuck as a zombie.
Madeline is taken to the morgue when she passes out in the hospital. When Ernest gets her, she panics about waking up alone in the dark. Now imagine if she hadn't been rescued, and had instead been buried or cremated.
Lisle hooks desperate old people in by showing them all of the benefits of the potion, then telling them about the downsides only once they've paid for and drunk it!
Lisle jabbed Ernest's hand with the potion to demonstrate its effects, as she had with Maddie. Does that mean his hand became immortal? When his body died, did his hand become undead? Is Ernest's consciousness doomed to spend the rest of eternity trapped inside a rotting hand?
By the end, Maddie and Helen are nothing more than dust and bones held together by a layer of spray-paint, but they remain conscious (and even after their bodies are shattered, the individual pieces remain active). This suggests there is nothing at all, nothing, that can put an immortal down altogether. Presumably even cremation wouldn't do it. When, billions of years in the future, the Earth is swallowed up by the Sun, will that finally be enough — or will their disparate particles (and those of everyone else who took Lisle's potion) remain conscious and aware of their surroundings, unable to do anything but burn for all eternity?
