To prevent your party-joinable NPC from getting a hefty HP and level bonus in LoB (Legacy of Bhaal) mode you have to set CRE flags bit 22.
This means that with NI I tick the Flag "EE:Ignore Nightmare Mode (22)", yes?
And, is it a good idea to put the following script block into my mod NPC's override script?
/* SoD: Remove story mode extras if NPC is not in party */ IF !InParty(Myself) CheckSpellState(Myself,STORY_MODE) THEN RESPONSE #100 ReallyForceSpellDeadRES("OHSMODE3",Myself) Continue() END