Ideally, we will need both additive and multiplicative. As you mentioned, the sky brightness can change with time during the night. So an additive term is needed. At the same time, a multiplicative term is also needed to account for the change in sky transparency, even if the photographer did not mix different ISO. And of course, in addition to mixing different ISO, it is also common for people to mix images of different exposure time. In this case, a multiplicative factor is also needed. So if at all possible, please consider an option that allows both an additive term and a multiplicative term.