how on earth could Helen afford to buy the potion years ago? She was only successful after she drank it. Before that, she got evicted because she wouldn't pay rent.
Explained in the film: Lisle says the price is different for each person, and Helen later says it cost her "everything [she] had, which wasn't much".
As a Wild Mass Guessing, Lisle probably also uses the money partly as an excuse. She makes it costly to the buyers because people expect something like that to be fantastically expensive, but the thing she's really interested in is the people themselves... remember, she's assembled quite an eclectic cast of some of the biggest stars and celebrities in history, presumably far more than we even see. She may have given the potion to Helen for effectively nothing just because she knew it would eventually involve Madeline as well, and she was eager to see how things would play out between them, like her own little personally-engineered real life Soap Opera.
What exactly is holding up the top half of Hel's body when she has the hole blown through her? Being insensitive to pain is one thing but she has a massive gap in her spine, is the tissue of the sides of a person's abdomen strong enough to support that much weight on it's own?
The gap's not as massive as it looks in the poster used as the main page's image, but it is pretty big. Probably best to assume that since the whole reason she's alive and mobile at all after acquiring said hole (besides the fact that the movie would be a little difficult to stage if she spent the rest of it crawling around on her hands dragging her lower half behind her) is a magic potion, essentially A Wizard Did It for real.