ok I am late into this debate. As I argued recently I still think Michael and Tom had a brotherly relationship, given that it was not a life insurance to be Mike's brother. It was a strange relationship, not blood related but quite close.

Michael was really small when Tom got into the family (how small is depending on the timeline of the book or the movie) and also Tom was several times the teacher of Michael(we know at least about the time when he was to be educated to be a Don, but I am guessing earlier too). They were both smart and educated so they could probably connect on many levels(like Tom guessed Vito's and Michael's plan), but still they were not equals. Michael indicated a lot of times that Tom is not a Sicilian.

Anyway, my biggest evidence for that Michael did not really blame Tom for the death of Sonny and the injury of Vito is that he survived. Michael never tried to kill Tom, what is more made him Don in his absence and put him back as a consigliere to the family. Michael did not have reservations about killing undeserving family members or friends.

I sort of understand why Tom is not responsible for Vito or Sonny. The one thing I never logically understood why Tom did not get killed for was the abortion. That happened under his watch. I though Michael would have killed anyone for that (and that was the only thing that he could not ever forgive to Kay) .

I still think that this (and probably Michael's condone of other mistakes too) is a decision of the author and the director. For the narrative, in the book and especially in the movie part II when Micheal gets more aggressive we need Tom, and his acceptance and understanding of Michael's deeds.