This is so awkward. I have been adding items (mainly Leveled) ad lib to many a Summon, and it works so well that i didn't see this one coming. Ages ago i gave the Dremora Summon a couple of bombs (4) to throw or trade, and recently i changed the quantitiy to 16.
Today I added some more at higher levels, but to my horror, 'tfh still showed the four -ILI entries. That is: with Heroes who had already summoned Dremoras themselves or may have encountered adversaries who did so. I consoled a summon dremora spell to a very young hero, and she had the new lists.
My guess is that once a dremora is summoned anywhere in the heroes life, its Inventory List is instantiated, and there's no way to remove that ingame. There may be tools that can do so, which i'm unaware of, but would really like to get acquainted with.
It's so weird that I can add anything to a Ghost or Skeleton, and it's there. The Dremora (and maybe others) seem(s) to be an exception. I haven't found a way to change the creature that is summoned by this effect, and i wouldn't like to have to restart my now L39 with his 240 Endurance or my 13 year old L83 Uber Bosmer once more just to get this minor update working.
I have already reserved an area in the mod for (a) statue(s) to the one(s) who help(s) me out on this one. It's a place that will spawn Dremora Summons at PCLevel+Luck dependant intervals. I hope that one day these Dremoras will have their max 80 bombs (L65+) in stead of the current max 4. And the scrolls. And the deadly weapons that suit their scripted per-level health / strength / agility increase.