Switched the death_management_events file back to the way it was before; namely, it now includes all events in the death_management_events vanilla file instead of only including the events that I had changed for this mod.
The events were working sporadically for me even within the same game; sometimes the Mourning modifier would be added, sometimes not.
It seems like the game would sometimes pull the events from the mod, but other times it would ignore them in favor of the vanilla events.
I had been hoping to keep the mod event file with just the events I changed so that it would be less sensitive to version changes and be more compatible with other mods, but sadly that does not seem feasible.

Updated to 1.4.0. Also fixed a bug that was causing an error; credit goes to Half-Lidded Bruiser for pointing this out.

Noticed there was a missing bracket for death of siblings over 65; reverted, fixed this, then re-uploaded..