1. Since I'm underage I shouldn't know anything about margaritas
Anyway, if I was 21+ I'd go up to them and say, "Hey, I was just wondering if those margaritas are good because I'm thinking of having some." IMO, its a tad corny but most girls I've seen don't just ignore the chance to offer advice. They'd pipe in and probably say good or whatever. Then say, "All right, I got to get some. Whats your names?" I feel that they should give me their names first before I give them my name. So that way if they say obvious fake s**t then its time to leave. I think since you started off on a good note, bring up actually interesting conversations like about the bar or the music thats being played, etc. Don't ask about women's height; they're not there to discuss life at a bar. They are there to get drunk and party. Then if all things go well and you feel that you made a great connection say, "Hey, we all got to do this sometime. Give me your numbers and I'll call you later, ok?" Again, if the night went well, they will give them to you. Then proceed to weed out the ones you don't want. Don't really ask them all out to drinks again. Its just an icebreaker to get their numbers.
2. Start getting closer to her. If she looks up, smile and look into her eyes. After the movie, proceed to make the first move. The girl all the time may just be waiting for you to make the move but if I was a person who liked the movie, I'd not like to have someone else get in the way hence after. If its a good movie at the end, she will probably have better emotions (I'd think) Then kiss her a bit and see if she kisses back. Proceed to get more passionate until you do the deed. Thats the only way how I'd pull it off. (heh, I said 'pull it off')
3. Given the fact that there are at least 3,000,000,000 women in the world, going back to someone who thought of you as an AFC is not the ideal choice. However I believe you can change. Just use your normal styles. Odds are that you will fail because she still harbors those feelings for you. Unless she is just a ho that doesn't remember you in which that case you're in like flint. I think that an old crush would be more of a last resort though.