So what happens if you cast a Death Curse while you are under the effect of a "ward off death" spell?
Not sure what you mean.
Are you suggesting that Kumori kept him 'not dead' like she did for Marcone's thug? I guess that is possible but it makes the casting of death curses a little less likely, IMO, and also takes away some of the potential fun of letting us see a partially botched necromantic resurrection.
I wonder what would happen if you used necromancy to make his dead body a homunculus for either his soul or his ghost? Basically zombie his body but then dump his soul/ghost back into it. It would probably allow him to retain his skill and power magically speaking (instead of having to relearn it like Luccio and Corpsetaker had to do after their body switching) and the use of a soul (or a ghost spirit) might work as a power source for the zombie body (allowing it to exist permanently and without a drummer or a beat to follow).
The way I had it playing out in my head is that Simon DID die (and cast his death curse) at Archangel (and he possibly took one or more death curses there as well...either from the traitor or because he WAS the traitor) and that Kumori escaped with his body and either brought him back herself or made a deal with the Kemmlerites to bring him back (thus explaining his providing aid to them during the run up to the Dark Hallow).