Sieghard is very much on board with killing Irene (because of the ambush) and he has no love for Sforza, though that doesn't rise to the level of "let's murder him in his sleep." I can easily see how one thing could lead to another though, where the results of killing Irene create a need to remove Sforza which in turn creates a situation where we feel the need to take control to enforce some level of stability.
Only if they find out it was us. You also seem to be assuming that Sforza dying means Carraciolo becomes undisputed ruler of everything Sforza ruled. There's no guarantee it'd work like that. For a historical example, when Alexander the Great died, his generals carved up the land he had conquered and split it among themselves.