Turnbull, before even reading the rest of your post the exact same thought came to my mind.

I never felt that Michael thought that he was treating Fredo unfairly or intentionally treated him unfairly. I think that Michael built a wall around himself with everyone. Distanced himself from everyone and would not let himself get intimate with anyone. However I think that Michael sincerely thought that by letting Fredo run some Mickey Mouse Nightclub, some Brothel, and giving him his own people, that he was doing the right thing by him. He was taking care of his brother. But Fredo was too dumb, too simple to see this.

Where I blame Michael is in his coldness. Again he was cold with everyone. But Fredo was his brother and not a smhat one at that. Michael should have spent more intimate time with Fredo, did things with him. Even let him sit in on the unimportant meetings.
If he had done that, had spent "more time together" with his brother, I don't believe that Fredo would have been so bitter.