1) It depends on the snake. Some will take f/t no problems, some need time (and tricks) to transition, and some flat out refuse to take f/t. I don't know the percentage, my advice if live isn't an option is to ask the previous owner/seller what they are feeding the snake beforehand. But do be prepared to need to feed live if you have a snake that just won't take f/t and starts to loose an unhealthy amount of weight (gotta do what's best for the snake).
2) I buy frozen in bulk for my 15+ snakes and it's a lot cheaper than getting f/t feeders from a petstore. In my experience with my snakes is that when neonates they step up prey sizes pretty quickly (depending on the species of course), so I'd get feeders in the next size up too.
3) I always throw away uneaten food. Just preference. You can always leave the feeder in with the snake overnight to see if it will take f/t that way, but definitely toss it in the morning if it doesn't eat as it would be hours into the decay process.