1. I don't think Vito was going to proclaim himself "boss of bosses". Where did you read this? By getting rid of Costello, he had already made himself the most powerful don anyway.

2. Nobody really knows for sure but I chalk it up to just intuitive thinking and action on the part of Croswell. I'm sure it was probable that Lansky and Costello were involved but at that point, neither one of them (or Lucky for that matter) were too concerned with LCN dealings. Lucky was deported and being closely watched by FBN and the Italian police, Costello had escaped an assassin's bullet and Lansky wasn't as concerned with the goings-on in LCN as many books and publications would make it seem.