Clemenza turning traitor would have been horrible and not very plausible. He was a real oldtimer who believed in the old ways. I would never see Clemenza turn.

And another thing which I consider somewhat sloppy writing is that clemenza promised the Rosato brothers three territories "after he died".

Clemenza died of a heart attack, no? How could he have known that he would die anytime soon and therefore promise the Rosatos territories after his death? It was not like he was suffering from any terminal decease...