Good info, a few comments from me:
Merisiel:
I took the Acrobat role in my 1st playthrough to midpoint and happy with Delay, but I am still undecided now that I finished A3S5 again.
Pro Acrobat: HS7 > HS6, Delay. I think 90% of people take the Acrobat role because of Delay, one of the strongest powers in the game. Alternatively, you *could* use Ilsoari Gandethus as an ally, and while it takes endless time going through the Sandport Devil dice procedure every time, he basically is the equivalent of Delay in ally-form - if you get him early in the scenario he will almost stick to you.
Pro Thief: Sabotage Location costs only 1 point (no need for Sabotage Barrier) instead of 2, Black Marketer > Item Collector (if you're into collecting weapons & armor at all).
And, afaik, some encounters in later adventures (and the main boss) can not be evaded, so no Hide/Delay on them.
Well, Acrobat is definitely suited for Quest Mode, carrying Ilsoari through literally (no, really) 100s of games could be cumbersome...
Kyra:
Completely agree with you. HS 8 for the healer is huge. My small party of 3 rarely used her healing powers sofar in the game (mostly because Meri & Seoni are 99% solo in their locations). Cure/Major Cure does the trick for Kyra herself for now, so I don't know. Exorcist may be useless if you don't face Undead or Outsiders. Dawnflower's Favorite though is nicer for the Exorcist, isn't it? Still undecided.
Seoni:
Took Celestial Sorcerer during my 1st run. Abyssal Sorcerer/Worthy Sacrifice: it could be useful in some situations, but I can't think of many instances where I would use it. But no matter if Celestial/Abyssal: I would first pump up Arcane Blast & Resistances.
Just some thoughts, fwiw.