You said that he was being jealous and not trusting her. AFTER he finds out you two start to go out he might think, "ElektroShokk must've been the reason why I felt a little distant towards her and there was something going on so I might as well lose trust on her." You even stated that during the times they went out you videochatted her like how a boyfriend would chat with their girlfriend. He basically thought something was fishy. I mean if you said you chatted with her 10am to 5am you could tell something was fishy and the fact that you said during video chat with Alvin inside the call while play truth or dare to which your crush said who your past crushes names were and you calling her name definitely sounds off. Then finally, he finds out that you guys go out, of course he would be furious and all his suspicions decides to be true. You even said that he said you guys betrayed Alvin which sums it up. Nonetheless you didn't make a move on her during their relationship, you guys were just simply a bit too close. Overall, your friend is actually a hypocrite not going to deny that. It could be that one reason why he tried to hit on 3 different girls was to make her jealous, a very common 'revenge' act.

Because it sounds like you two clicked more than her and Alvin, I think you guys are better off together. And since it sounds like he won't be any place to forgive you even if you decide to break it off, I suggest go for it. Your relationship with him won't improve in a short matter of time or it won't be the same as before judging by his personality even if you do decide to break it off. If you go for her, at least you gain a diamond but lose a dirt instead of losing a diamond and receiving a common dirt.