I'm sorry, I didn't mean to discredit your fix or your skill in any way, but as you stated "MaNGOS is an educational project" so I wanted to show you another way to fix this which I _think_ is simpler. Anyway to explain this better:
Unit::HandleDummyAuraProc is not triggered just by dummy auras, it is triggered by all of these auras (not sure all of them, i might have added a couple in my repo):
SPELL_AURA_IGNORE_REQUIREMENTS
SPELL_AURA_MOD_DAMAGE_PERCENT_TAKEN
SPELL_AURA_MANA_SHIELD
SPELL_AURA_OBS_MOD_MANA
SPELL_AURA_ADD_PCT_MODIFIER (this one is the one that is used to trigger the serendipity removal)
SPELL_AURA_DUMMY
EDIT: To make it work you have to edit spell_proc_event:
INSERT INTO spell_proc_event
(`entry`, `SchoolMask`, `SpellFamilyName`, `SpellFamilyMask0`, `SpellFamilyMask1`, `SpellFamilyMask2`, `procFlags`, `procEx`, `ppmRate`, `CustomChance`, `Cooldown`)
VALUES
(63735, 0, 6, 4608, 0, 0, 0, 0, 0, 100, 0);
EDIT2: Swiftmend is handled there because it has to modify m_healing which can't be done from proc, so I think it should be ok.