1. If a seller does not specify that a BP is feeding on live, pre-killed, f/t, etc. is it safe to assume that it is on live?
Typically yes. It's very true for hatchlings. Your best bet is to ask the seller if they will try feeding f/t for you before you buy the snake.

2. What are common / accepted questions to ask a seller about a snake's feeding habits?
What is it eating, mice or rats, what size, live or f/t, how often do they feed it, how many times has it ate (if it's a baby), does it go off feed, is it a picky eater. That's everything I can think of right off the bat.

3. Is it possible that a breeder would be willing to attempt switching a snake to f/t or even pre-killed, while still in their possession, if I told them I wanted to feed f/t and had put a deposit on the snake? (not sure how the protocol on this goes).
Yes, I would say most breeders would at least try if they have that size of f/t available. I have done it, but only for people that specifically ask me to. It's much easier to feed babies live so I'm not going to take the time to feed them f/t if I don't need to.