Don Mikey and Puppeteer gave you good answers, BofB. I'll try on #5:
Michael knew Zasa was a lowlife gangster, period. He said so, when he described Zasa as "a pazzo (crazy person), and as "pure muscle"--in other words, not a "legitimate" businessman like Michael thought he had become. I think Michael was trying to distance himself from Zasa and everything Zasa stood for. So when Vincent killed Zasa with Connie's and Neri's approval, Michael was pissed because that killing would have consequences--"pulling him back in again." Probably he was also pissed because they acted while he was too weak to stop them, a reminder of his increasing frailty.