[Rivière]

Based on what you've written, it comes across like A is a little jealous that you're moving forward, and past him. Given that he moved on and has started seeing somebody else, he may not have expected you to do the same and expected you to be pining after him. Perhaps he didn't know how to take the situation and liked receiving attention from you, regardless of how little it may have been. Of course this is all speculation and may not necessarily be true.

If A wanted to keep you as a close friend, or even just a friend in general, he'd have put more effort in. Not just talk to you/show signs of interest now that you've been taken off the 'I'm available' shelf.

Personally, I don't see why you should ask A if it's ok if you should go on a date with his friend. You're all consenting adults. You don't need A's permission just because the person you're going on a date with happens to be one of his friends, and a not-so-close one at that. Even though you don't want to make him uncomfortable, you're not A's property. You're not anybody's property. You shouldn't have to be asking anybody if something is/isn't ok to do. If you want to go on a date with this guy, do it, and enjoy doing it.