If Tessio knew Barzini would not be at the meeting then no show Barzini knew Michael and his people Tom, Neri were all going to be massacred that Tessio was doing his dirty work for him

Barzini could have had his people take Tessio out then and there unless he may have thought he could be of help in rounding up the Corleone clan and their operations before whacking him
He is the only one left standing Who is going to challenge him?

Greene had already made a deal with Barzini to keep the Hotel I reckon they will also offload Fredo Why let him keep banging cocktail waitresses two at a time!

Little boy Anthony when he is a man he'll come for revenge

I reckon Corleones totally annihilated including Fredo and Anthony