1) I saw that some people figured out the identity of the devil's trumpet a few pages ago and were worried because, well, it's super poisonous. Which is true! But it does have some real world medicinal uses (including a cough suppressant), which is what Jekyll's alchemical brew is based on. This specific brew is itself not real (in case you couldn't tell from the cobwebs), but real world alchemists were fond of using poisons and other definitely-not-physician-recommended substances for medicines and/or elixirs for eternal life.
In the TGS world, even the most fantastical of alchemical formulas are totally legit, but obviously, please don't go messing around with devil's trumpet in real life, even if you have a whole bucketful of holy water.
2) Also, Hohenheim was a real life alchemist, though his accomplishments have been embellished upon for this comic--I'm not trying to create a Fullmetal Alchemist shared universe.
3) I'm not sure how much Frankenstein trusts Jekyll's scientific expertise, but she would absolutely be willing to poison herself just to prove a point.