I found it hilarious at just how much of a grovelling little creep Mark was after the alliance was exposed. "It wasn't meeeeeeeeeeeeeee". All his dramatics about how he fought for Ash and it distressed him so much to do it. He enjoyed being part of a secret Power Alliance as much as Chris and Ashleigh did. But that's Mark all over isn't it? Everyone's "bezzie mate" until anything goes wrong, then he'll throw them under the bus and try and save his own skin.
As for the Christopher situation, fair enough if he doesn't fancy him back, but he hasn't exactly been clear about that has he? He's done the whole "I like him as a friend" thing in the Diary Room, but he's told Christopher they are on the same page (which is a conveniently vague statement, so he can't be called out on it) and the other housemates have said he's never told them he doesn't like Christopher in that way. I think in that sort of situation, it's fairer to be clear to the person who fancies you that you don't feel the same way, not string them along, take advantage and make them think they might have a chance. That's, at best, incredibly selfish. But then that wouldn't be out of character for Mark would it?