Kezef
If it did not happen, I am going mad. This was with the mod as it was a month or 2 ago, in the scenario where you play Falmer and the AI decides who is at war with whom. I have updated since, and I cannot load the old save games from then. I will let you know if I can recreate it.
I believe the bug you mention was a problem at one point, but was fixed before the last release. Your saved game might have been from just before it was fixed.
I like the idea of Trenton getting some love, too. He can be reunited with his staff! That might even be a super cool way for the Illians to speed up a ritual, although i don't think they need it right now (at least as a player - the AI is never ready for some glaciers).
I think limiting it to Arcane units is a good idea, as it would help imply that only those deeply interested or attuned to magic would be interested in attempting the trials. It could otherwise give an increased success chance like the spellstaff.
The Bless to Potion change sounds amazing, it fits the sphere much better, and it also cements a creation mage as an ideal city booster. I don't know how frequently the effect procs, but i suppose what effects the potion will have should be balanced around that; how frequently it can be given to other units you train much like when 'distributing' spellstaves. Personally i think healing or curing disease-type promotions are ideal, but it sounds like other possibilities are open as well.
I went ahead and made Creation affinity randomly grant a Potion of Restoration, and also improved the Potion so that it fully heals the unit, lets it cast and attack again, removed any negative effect promotions(included Diseased, Plagued, Rusted, etc), and restores any positive promotions (except equipment) from the unit's xml defined in case they have been lost. (The Potion can thus undo The Crucible or the Force IV Purge Magic spell's removal of Channeling promotions.) The Potion promotions however now may randomly expire. Such powerful medicine has a shorter expiration date than a Healing Salve, Potion of Invisibility, etc.
I went ahead and let any arcane unit use the Cave Trials, but Arcane helps survive it. So to Spellstaves, potions, etc, although those items are lost in cases where the unit might otherwise have been killed.
I guess if I'm sticking most closely to lore none of Kylorin's Students should be available for resurrection, as their deaths should have all been long ago for their Sluaghs to leave Erebus for a heaven, hell, or neutral vault and be processed into angels or demons. I guess resurrecting Trenton shouldn't be too much harder than that. The trauma of seeing whatever killed his people should definitely make Trenton Crazed though.
I've decided to go ahead and make the Imprimatur council members their own heroes rather than sharing a unitclass. I've made some custom art for Herve (a Svartalfar Druid but in more Ljosalfar colors), Carnivean (a Lanun Ranger with darker pants and shirt and glowing eyes), Asmoday (a Profane but with the same devilish mask as Sarabride), Trenton Majosi (a Charlemagne scenario Light Spearman with bluish skin and gills slts), Soqed Hozi (an Elohim Adept with the trim of her dress changed to red, longer sleeves, and the Order symbol printed on her book and as a necklace). For now I'm just using the normal Whisperer art for Wode, Ljosalfar Worker art for Majen, and Djinn art for Gastrius. I'm not sure if I'll end up using the Gatrius unit anyway instead of keeping it a Great Commander, as it should canonically only be able to use its abilities when attached to another unit that it makes become the Casswallawn. .
That got me thinking about some other Imprimatur Council members. I was first thinking Tamesis's sluagh could rest at the Mirror of Heaven, but canonically he did not actually die there. He was broken for the Ritual of Endless Night but survived with his powers intact but uncontrollable. He then spent the rest of his days in the Spire of the Sun, a shrine where he confined himself to protect people from his overwhelming power yet let them visit him to give him worship as a living god. I was thinking that the Spiral Minaret art could work well for the Spire of the Sun, but when searching for it found that BtS's Shwedagon Paya works even better. Thus, I think I'll be including another Unique Feature that is a source of Sun mana too.
I changed the effect of Crown of Brilliance so that in Tamesis's case specifically, it damages friend and foe alike. Tamesis's uncontrolled power also auto-razes like the War Machine. I just made a reskin of the Malakim Immortal with light yellow skin to be Tamesis.
I'm also thinking of adding Velgyr's sluagh on a new unique feature, but am still deciding on the name and art. Velgyr himself may just use Luchuirp archmage art. My first thought was to use the Celestial Compass art for the unique feature, as it sounds like his mysterious Astrolabe, but it does not look right in the game on its own. I think it might be great as part of a larger model, but I don't know how to combine nif files. I might use the Observatory but am leaning towards the Public Transit art from BtS. We know Velgyr is severely autistic and avoids people but built himself a model city with some very realistic golems (some which pass for human, some for butterflies, some for dragons). It might be appropriate to make his Unique Feature a Clockwork City with moving model trains.
As far as the other students of Kylorin go:
It probably does not make sense to place a sluagh of Leucetios anywhere and she is eternally young and probably immortal. (My headcannon is that the quick drop in life expectancy after the Age of Magic is due to Os-Gabella capturing the Mother of Nations in a cell near Nemed's in order to prevent her heroic long lived children from enriching the human genepool.)
Os-Gabella and Perpentach are of course leaders in the game. If could give them avatar units (The Red Jester sort of already is one), but those would probably be limited to scenarios, not linked to improvements.
Paimon is already a Grigori hero.
I don't know what kind of improvement could be linked to Kezef or Mikel Dylantyr.
Jenkin's race is immune to time, he is probably not dead in order to be resurrected, but I think he returned to his own world.
I could consider adding Badb's Blizzard, but that feature would have to move around at random and Badb himself is said to be still alive but trapped within it so resurrecting him wouldn't make much sense.
I could place Laroth's sluagh on Bradeline's Well, but I'm thinking he is much too important to let us simply resurrect him like that. He is not dead anyway, just biding him time in the realm of the dead until he can gather the forces to become its new god.