I'll take a crack at # 1...

If the reasoning of the Corleone Family Brain Trust was correct in that the Sollozzo-Michael meeting was just a Sollozzo trick to buy him a few days time before attempting another hit on Don C., then Michael's killing of The Turk was their only option. Your suggestion that they could have used their newspapermen and judges, etc., requires time, and the Corleones, rightly or wrongly, felt that more immediate action was required.

That being the case, Michael was really the only one who could carry out the hit. With the scheduled meeting, he was the only one who could actually get close enough to Sollozzo to kill him.

Michael points this out to Sonny in the novel, and adds "I don't think you could get anyone else (willing) to knock off a police captain. Maybe you would do it, Sonny, but you have a wife and kids and have to run the family business until the old man is in shape. So that leaves me and Freddie. Freddie is in shock and out of action. Finally that leaves just me. It's all logic."