Graft Flesh was last printed in Lords of Madness. Everything that uses the new (stupid) grafting rules relies on a different, more specific collection of feats.
Ok, got it. So Graft Flesh is an Item Creation feat, as are all the other Grafting feats. As such, Binders can grab any Graft feat they want binding Astaroth.
Hmm, good point.
Also, somatic components have nothing to do with how your body is controlled, only hand gestures matter.
I was attempting to make a RAMS justification for something that is clear by RAW. Despite your arm being incorporeal you are still (mostly) corporeal and thus capable of being grappled, which invokes the rule against casting a spell with somatic components.