Welcome back, Frank. Been a long time.
As you probably know, Richard Castellano was approached about appearing in GFII, but he wanted too much money and script control for Paramount's liking. So, FFC and Puzo wrote Clemenza out of GFII and created Frank Pentangeli as Clemenza's successor after Pete died of a "heart attack" (or maybe not?).

So, we're talking about two entirely different characters. Of course Richard Castellano--had he appeared in GFII--would have been given an entirely different personality and script to follow than Michael V. Gazzo as Frank Pentangeli. The concept of Clemenza surviving into GFII, and Castellano playing him, has been discussed many times here, and many people believe (as you do) that Clemenza would have remained totally loyal to Michael. But, Clemenza didn't survive into GFII, and we got a different boss of New York: Frankie.

IMO, Michael V. Gazzo makes GFII--Everyman as Mafia boss. He richly deserved the Oscar nomination he got, and I would have been delirously happy had he won. He's my favorite character in the entire Trilogy.