Pete, I agree with your second and third points:
I'm guessing that, after Vito turned Solozzo down but Sonny showed interest, Sol had to get permission from all the Dons to attempt to whack Vito. He probably had to promise them a piece of the drugs action with the assurance that Sonny wouldn't go to war because Tom would convince him to make the deal. Even after Sol was whacked, the Dons probably saw the potential of drugs and either began or planned to pursue drug trafficking. They needed Vito's judges and politicians as much as Sol did.

And, yes, Mob wars are always about money and power. Barzini saw not only drug money, but the opportunity to become Numero Uno.