>>1 none of us can choose for you, make the choice that you'll least regret.
Does she act romantic around you? Is there a possibility that she might have feelings for you? It's okay to be curious and you could be bi. If you think it's worth risking your friendship, and if she's a true friend then that shouldn't matter, then confess but remember to say that if she doesn't feel the same way it's okay, tell her that you just wanted her to know.
Make sure that this ex boyfriend actually wants a relationship before you accept. He could be just using you so be careful!