Excellent post, and welcome to the boards.

Some of what you say has been covered in other posts. For example there has been much chatter about how badly Tessio would have fared in the Barzini organization had his plan worked (dead within six months).

I think Tessio was driven by hubris, an enormous underestimation of Michael, and greed. The hubris was in believing as a senior capo in the Corleone family that he could arrange a meeting between Michael and Barzini without first consulting Michael. I believe this showed he believed himself better than Michael simply because of his seniority and past association with Vito. The correct thing would have been for him to come to Michael and tell him he had been approached...wha did oit mean, and if there was a meeting a "negotiator" would be held to insure Michael's safety. He did none of that.

This ties in to his underestimating Michael. When he tells Hagen he always liked Michael, and that his betrayal was "business," what he is really saying is he didnt believe Michael would succeed in running the family, and he was jumping ship.

The greed is obvious. Barzini had to have promised him not only his old territories, but a ton of money and a high position in the family if he could deliver Michael.

As for what may have happened to Barzini, I think he was way too smart to have been anywhere near the scene of Michael's assassination. Barzini's "arrangements" were to put Michael in a car with "his people" and take him to some remote prearranged location where he would die. My guess is as soon as Michael drove away, Barzini would do something similar to what he had Solozzo do when he thought Vito had been killed. for instance, kidnap Hagen, or Clemenza, deliver the news, and tell them the Corleones were finished, and their only option was surrender.