I guess we could rephrase this question as follows: Would Michael have pulled off these killings while Vito remained alive?

I don't think so becaue Vito was certain that a traitor would
emerge to lull him into coming to a peace conference. This suggests that it would only happen after Michael became Don, and to the world, even though Michael was the heir apparent, there is no question who the Don was. When Fontaine signed the contract for Vegas it was for his godfather, not Michael.
At best the word was out he was "semi-retired."