Your fix would work well for me, if you flipped it. Sovereigns are the source of magic, even more so now that essence is a yes-no trait and not a spendable stat. It should be their choice of Life vs. Death magic that counts.

Re modding, it just seems like the system should be as open as possible without being a mess for coders &/or AIs. Life-vs-Death is a trait that might be 'binary' in the base game but shouldn't be all that hard to base on a list of 3+ names that refer to discrete sets of data/methods for the sovereign objects.