Both questions have a lot to do with Marcello's character flaws:
1) Though not explained in the film, they were the backup hired by the Fascists because they knew that Marcello would be unstable in carrying out the task due to his feelings towards Anna (which is why he was followed around too closely). That was made more than evident when Anna was screaming towards Marcello and he did nothing, which hit the nail on the head as to how much of a coward he really was, as well as being a pawn.
2) Again, it had a lot to do with Marcello's cowardice in that it was his way of getting back at the molestor alongside the timing of Mussolini's fall.
That might help you, hopefully, though can be some other observations about this.
Thank you both for your responses. It's been over a month since I have seen this film, but I still can't get it out of my head. The staying power is incredible.
Re question # 2: When Marcello is confessing to the priest that he murdered Lino, he talked about his penance being to commit another sin - or something like that, something about one sin balancing out the other. At the end of the movie, when he discovers that Lino wasn't dead, my thinking is that he felt that he could no longer justify the murder of Quadri - because there was no first murder. That is, Lino, by living, nullifed Marcello's convoluted motive for killing Quadri and therefore somehow indirectly should be held responsible for Quadri's death. I'm not saying this makes much sense - especially since Marcello couldn't bring himself to go through with Quadri's murder so was only indirectly the cause of his death. But I think that's the connection.
Also he sees what happened w/ Lino decades earlier to be the cause of his drift toward Fascism; so, as he sees it, his betrayal of Quadri is Lino's fault. (A coward's logic, of course)