Hi J.,
The poop issue has been an obstacle for all 3 of mine. My first was fine w/ peeing by 2 1/2, but wasn't completely poopy-trained until 3yr5mo (gasp! I know.) She was and is extremely advanced intellect-wise, and was finally convinced that her bm's could no longer fit in pull-ups/diapers!
Now, I'm dealing w/ boy/girl twins who are 3yrs 7mos, and seriously I think they look at each other and think "well, he/she still goes poop in pants, I can, too." My boy's bm's are looser, so he'll go every now and then in potty, but he'll be so busy playing and since they are softer poops he'll go and not know I think! My girl's bm's are harder and less frequent, and I remember being like that around 3 or 4, so she still has to rely on the tried-and-true squatting position that she's done her whole life, she'll tell me that poopy won't come out on the toilet. (I'm modifying her diet, and provide books to read while on potty, searching for other solutions!)
As you can see from my experience, each child is different, but like my first, it will happen...someday. But these twins aren't going to Disney World anytime soon as long as I'm still buying diapers/pull-ups/wipes$$$$$$$$$$
About underwear...when your daughter poops in them, and this might work if they're a fav pair, throw them away!!! Let her see you put them in the trash. This worked for my sister's boy, who loved his Batman underwear and one day asked her "you're not going to throw these away are you?" Obviously, he was 3 1/2, advanced language. She said, "If you don't poop in them I won't." That's what finally completed his poopy-training. My son was a bit remorseful seeing his poopy-Mickey Mouse ones going in the trash, but we'll have to try again!
I hope to find more helpful hints from your responders!
Peace,
K.