The 2nd and 3rd points in your 7th reason only imply that Avery was really dumb about committing murder, not that he was incapable of it.
I binged watch the entire series in about 48 hours over the weekend and then read some articles afterwards. I think that Avery did it, but the county and state conducted an unfair trial. It also didn't help that everyone in Wisconsin knew his background, so there was a low probability of finding an unbiased jury.
Also, I'm not entirely sure how this works, but Brendan should have plead insanity or something. The kid was obviously manipulated into a confession by the prosecution.