Jidokwon: Imoen will be my primary arcane caster. I'll likely use Aerie for my secondary. I'm considering doing this run with monk, Kaldorn, Imoen, Jaheria, Aerie, and Mazzy. I might do Nalia and Amonen instead of Aerie and Mazzy.
For keeping Imoen, I'm going to try spawning her later version, but I want to make sure it doesn't trigger the spellhold dialogue. Even if it doesn't, I imagine I might run into some problems in Spellhold when the dream kicks my party members or when I am supposed to rescue the real Imoen.
Another option, though a bit intensive, is to take one of the NPC's that gets added back in by a mod and use Shadowkeeper to change her into Imoen. Replicate the stats and everything else. When you get the real Imoen, make note of the stats, drop the fake Imoen, then use Shadowkeeper to copy the stats into the real Imoen. Wouldn't have to worry about breaking the game that way.
I suppose if you have any experience with doing mods on characters, you could create just a small character mod that replicates Imoen and drops her somewhere else in the city. The game would treat her as a completely seperate NPC so it wouldn't break anything. I worked through a character mod about a year ago, they aren't that complex to make.