I don't believe that the regular Untold cast can make use of Kupala's Soul. Kupala stated clearly that she was placing the power in the Highlander's Spear. I think those increased stats were just gameplay mechanics. Still, being able to defeat Etreant is pretty impressive, so it's not that huge of a gap. I'd say that Etreant should be placed at "At least Country level", just to be safe.
The thing I'm curious about is the Untold 2 cast. They were not only able to defeat the Overlord, but they were able to injure Yggdrasil Core in a similar manner to how Visil's Serum injured Primevil. That might make them equal to the Untold 1 cast, and the Overlord equal to Etreant. However, what of Bertrand? We know he drasically powers up as Demi-Fafnir, and it took a Black Guardian Fafnir Knight to stop him. This same Fafnir Knight defeats the Overlord, meaning?
Kupala's Soul Highlander should be equal to Primevil. However, Grail of Kings Fafnir should be greater than Yggdrasil Core. I agree with you there.
Massively Hypersonic speed seems fine to me. As it's a turn based game, our only real feats involve dodging lightning. I'm not sure if the Wyvern uses natural lightning, but I am certain that the Overlord does (Crashing Thunder skill), which scales to the Untold 2 cast.
M.I.K.E. was still able to put up a good fight to the Untold 1 cast even after they had defeated Etreant, meaning that he definitely scales. As for M.I.K.E. with Gungnir, that's pretty obvious. Multi-Continent level.
Classic Mode characters, I have put some thought into, and I believe that they should scale to the Dragons (Great Dragon, Storm Emperor, Blizzard King, Wyvern, and Fallen One). During one of the post-game quests in Untold 2 (not sure if it happened in the original or not), the Storm Emperor comes down from the Labyrinth and destroys High Lagaard's west ward. As High Lagaard is made up of six wards (North, South, East, West, Slums, and I forgot the name of the last one), Storm Emperor should reside at 1/6 of City level. Great Dragon and Blizzard King are comparable to Storm Emperor, and it took all three elemental dragons to seal the Fallen One, meaning that Fallen One is probably 1/2 City level. This probably makes Fallen One Low 7-B, which will likely scale to the Classic mode characters.
Anyway, I have no idea who to ask about for help. You are the only person I know on this wiki who has even heard of Etrian Odyssey. Still, in any case, I believe that if we are to recieve some good input, we should get an unbiased opinion, meaning we get someone who doesn't know the series to analyse our arguments. Got any friends on the wiki who fit that description?