1) Michael didin't think there was a traitor in the family until AFTER the hit attempt

2) In that scenario, what were Fredo's associations with Ola & Roth all about?

3) And who killed the hitmen in that scenario? Someone else would have had to have been in on Michael's "plot"

4) Much too risky. Bullets flying all over his bedroom where his wife sleeps and his children come to play with their toys