How on earth could Helene dying of natural causes possibly prevent her reincarnations from being born? Whether or not she dies of old age or on the chopping block, she's still dead.
Fridge Horror: Helene's future reincarnations along the "death" path speak to her, pleading to be allowed to live, which means potential incarnations possess some sort of consciousness. If her survival meant that an entirely different set of reincarnations (on the "life" path) would have come to be as a result, then what happened to all those perfectly innocent women when she chose to die?
Those women shouldn't have existed in the first place. The fact that the Devil wouldn't mind her staying seems to indicate he'd be able to pick the women who follow her.
Also, what's to stop Quintis from mucking around in the past? He has something like 700 years of scientific and cultural development to draw upon. At the end, Diana and Bald Scientist Guy should be lamenting Quintis' fate while sitting in a giant crystal spire or something. And why does the Devil think Quintis belongs to him? Time Travel isn't a sin, though Pride is one of the Seven Deadly Sins, and Quintis is a Smug Snake to end all Smug Snakes.)
Hypnotism and past life regression might be considered dabbling in the black arts, the Devil tells Quintus such when he calls him by name.
After Quintis explains the paradox, Tom adds "Either that, or I'm wrong."