Official rulings by Konami. Apparently, since Venom requires monsters on the field as Materials, those monsters in the Graveyard are no longer considered valid Materials, so they can't be summoned. Same with Defusion. I know, it doesn't make any sense, but it's an official rule, so... nothing to do about it, I guess.
It does make sense. Starving Venom's listed Fusion Materials are 2 DARK monsters on the field. It means, it requires them on the field. Once they are in the Graveyard, they are not valid targets for Superior Polymerization or De-Fusion.
Oh and I ask for the rename of Superior Polymerization since in game is still listed as Transcendental Polymerization.