That is a very interesting idea you have, it’s just a little tricky to work it out right, but if done well it could be very entertaining. It reminds me of the joke, “How do two porcupines make love”, answer, “Very carefully”. Yes, if Leah is inadvertently bitten by her vampire imprintee, she will die, sort of putting the imprintee into Edward’s shoes, except the situation will never change, and like Jacob and Nessie both will become immortal, which means the venom becomes an eternal threat. Also both of them will have to deal with what is, to them, the offensive odor of each other. I think Leah would have an easier time with that than her imprintee, the power of the imprinting bond and all.
Sure, I think you can do anything with a vampire’s special talent, but making it believable is another matter. Being venomous is one of the weapons that a vampire has. The ability to turn it off does not seem to be a very powerful talent, and the only function would appear to be facilitating their relationship, which makes it look contrived and a little too convenient. The closest parallel I can think of is Bella and her “silent mind” and the way that initially seemed to be useful only in advancing her relationship with Edward, but she eventually finds out it is more powerful. I’m trying to think of how being able to turn the venom off could be useful in other ways, but I’m coming up with a blank.
"It will take an amazing amount of control,” she mused. “More even than Carlisle has. He may be just strong enough…the only thing he’s not strong enough to do is stay away from her. That’s a lost cause.”