First of all a few things few seem to focus on. Death never said he could kill god at will. He merely said one day he will reap him and that one day god will die. While this means Death MAY be able to kill god, it's not certain. Maybe when god dies, either from age, injury, or by choice, Death may have to be involved (curious what that would mean now that Death is dead).
However, even if Death can kill God that in no way means he is more powerful or even equal to God. Demons have killed angels, they are certainly not more powerful than them. Lucifer is more powerful than Gabriel but Gabriel seemed pretty sure he had some chance of taking Lucifer down. Death may be in the same league with god just not his equal, and he certainly seems to be above pretty much everything except God, the Darkness, and possibly Godstiel and the arch nyphilum. It's possible he is god's equal in power but not in other ways (like creating universes).
So the whole A>B>C argument doesn't really follow.
It seems that Death may be able to kill anything that can be killed, or at least has a hand in it. It's possible he can't kill the darkness because in killing her the universe will end. I think her not 'knowing' Death has some greater symbolism. Perhaps because she is older than Death she nullifies his powers like Eve and Raphael did to Castile.
Death claims to be at least as old as god so god wouldn't have this ability. She and God can kill each other but it results in the end of the universe. Why Death can kill god but not her, whether death can kill God under what circumstances, and why God can die in the first place when amara can't is pretty much speculation. But it seems that somehow Death and the darkness can't interact and so even if he were still alive I don't think he could fight her.