Yepp, it`s what I described. But I want to add some small touches to your setup
Red dots are id_Edgy matchbox shader. It finds any pixel with a zero and replaces it with something like 0.00001 or as close to 0 as possible. Divide/Mult by 0 is not a good thing in this case.
Yellow dots are CC nodes that are linked to each other, we will need to alter only the gamma setting to help blend between original grain and generated grain where comp matte is half transparent
PS How does the temporal sample mix part of your setup work?
PPS Some shots works better when performing all of math in log, not in linear