S.M.
Uh... underwear is not a choice.
I would think if it like this: You wouldn't let her go play in the snow wihtout a coat, don't let her not wear underwear. I'm not saying she would like it, but it's not really up to her, no?
I would lose the diapers. If she begins to sense that there is a choice, she will always pick the ease of diapers. It sounds like she is doing great, just get rid of the diapers in and out of the house and insist on underwear.
If she is a young 2, you might have accidents - that is one of the drawbacks of training them young. But I am guessing that she won't like it and stop soon enought. She seems to have the control, so it's a matter of remembering to get to the potty. The diapers might be confusing her, because she feels the underwear and it feels like a diaper that is dry. You've got to be consistent, monitor her liquid intake, take her to the potty frequently, and always bring a change of clothes.
I also suggest having her help clean-up her accidents. Unfortunately, some kids don't mind and love to help, but some donn't want to, and this can be a big motivator too.