Okay it was definitely option 2. I removed the remove on death from the debuff and it restored mana to the caster as expected. Only drawback is now the corpses have the debuff showing on them, it will still be removed at the end of combat, and maybe someone knows how to make it remove when triggered.
For anyone wanting to make the skill work in file "assets\data\Effects.gd"
function e_t_devour
line 2178 change
rem_event = [variables.TR_COMBAT_F, variables.TR_DEATH],
to
rem_event = [variables.TR_COMBAT_F],
as for making the skill name and description translate in file "localization\en\main.gd
Lines 694 and 695 change
SKILLDEVOURSPIRIT = "Devour Spirit",
SKILLDEVOURSPIRITDESCRIPT = ...
to
SKILLDEVOUR = "Devour Spirit",
SKILLDEVOURDESCRIPT = ...