Why kill Michael?
I mean, think about it. Jimmy and Shawn know each other, their sons would know each other too (actually they do, because Michael screams to Danny: It's Mike! It's Mike!). So why he decides to shoot him?
Because he witnessed Danny ending Albanians? So what? They're on the same side, Michael would never inform on Danny, if not for the sake of old times, then because Danny or Shawn would threaten to kill his daughters. He was a reasonable man, I'm sure he'd comply. Besides, what does he care - let them mobsters kill each other, it's no business of his.
Maybe Danny and Mike aren't the best of buddies, but they are no strangers too.
Or maybe we should believe Danny wanted to do it, because he was a hothead and stoned? Yeah, but kill someone you know and would expect them to keep their mouth shut? Kill them without even talking to them first? Without checking what would be their intentions? Really?
It just doesn't seem to work 100%. And it's a major plot fragment, it basically triggers everything that follows, so I'd expect it to be flawless...