Try suggesting a double date. You, one of your friends, your target and her friend. Obstacle eliminated. As for the girl rule thing you mentioned... just explain that you like your target, she likes you. The other girl doesnt stand a chance so where's the logic in everyone being upset. Besides if she hooks up with your friend, everyone's a winner!
Now, conversation topics. Im guessing common interests have been exhausted, or else you wouldnt be asking. The typical thing to talk about is favourites; holidays, foods, drinks, teams, sports, books, films, music, games, phones (maybe not

), celebs (girls love gossip). Oh also, a game I play with girls that I find is fun and demonstrates confidence is the celebrity game. Who would she sleep with etc, you name one after another, she says yes or no. Ask her why, stem a conversation from it!
I hope this helps dude!
GrifterUK