1. Ifyou havent noticed, Tom comes back in. Why exactally he was "out" is a matter of debate for some people, there are numerous diffrent reasons.
2.Tom and Micheal both know of it, but they didnt really want people like Geary to know right then.
3.Again, Tom came back in. Micheal knows it was somebody on the inside, but he doesnt know exactally who yet. The other Italains probably wouldnt approach Tom because, like you said, he isnt a Sicillian. But neither is Roth.
4. There is a lot that can be considered "towards the end". Are you talking about Mama's funeral where Fredo is crying his eyes out? I would say Tom feels sorry for him, partly because he lost his mother(and a lot of people assume Fredo was her "special" child) and partly because he knows w/ Mama gone, Fredos days are numbered.

I hope this has helped, and Welcome.