Ernest becomes a great surgeon again because his right hand was affected by the serum, and he quit drinking.
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.
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.