1. I'd venture that it'd be a highly-convoluted, but possible, process that someone with particular skill and focused intent could pull off. They'd presumably have to start by being notable enough to become a fiend directly upon death, as opposed to a petitioner, using something like a bargain with some other evil force or a soul anchor to retain memories upon passing. That'd be a whole quest in of itself. After that, they'd have to pull something similar off, using even more-esoteric methods, possibly involving bargaining with the theoretical highly-powerful nightshades or some death deity, or alternatively involving some complex arcane necromantic working, in the depths of the border between the Plane of Shadow and Negative Energy Plane. It's a feat I could see as something a notable campaign villain pulled off in its backstory (or a post-campaign goal for a PC in a successful evil campaign), but I wouldn't count it as something reasonable for an ordinary individual to try doing.
2. None known yet to my knowledge. Just rumors so far. I'd like to see more about them, though. Nightshades are nifty.