I think Michael's turning point was even before the Solozzo scene.

Remember at the hospital, when Michael realizes that a hit is about to take place against his father. He tells his father "I'm with you now. I'm with you now." And Don Vito is smiling as tears trickle down his cheek.

THAT point is when Michael realizes that he is and will always be a part of his father's family.

How could Michael come up with the idea for the Solozzo hit if he wasn't already there?

Not that the Solozzo scene didn't have a huge impact on Michael. It was his first killing (except maybe for some unkown, distant killings in war.) But by that time he was already on board.