1. That is a good question. Likely a highly debatable one, in fact. Thought I think Gotti would have done it anyway for reasons I'll give in reply to number 2. If he'd had to deal with Roy in the aftermath then I think he would have found a way to try to do that.

2. There is no chance Gotti would have been picked to be Underboss. At that time there would have been two dozen guys more qualified than Gotti, based on loyalty alone (which is the only justification for Bilotti.) Gotti actually may have made a great number two, but there's no way he would have been picked by Castellano for good reason.

I wouldn't read too much into Castellano's not attending the funeral, that was just some icing on the cake that Castellano gave Gotti to sell the hit to the rank and file. At that point, Castellano was going down anyway because Gotti didn't want to turn over the Ruggiero tapes, didn't want his crew broken up and had legitimate reason to fear that his own life was in danger. Once Dellacroce was cold, the wheels were in motion and I'm sure that was the plan all along.

Prior to the Castellano hit the assumption was that Dellacroce was keeping Gotti alive. As it turns out, he was actually keeping Castellano alive.