Right now, Yukako and Rina can be fully corrupted. In v0.2.5, Ayumi will also be able to be fully corrupted.
The Holmdow quest isn't vital to the game, and you can get it's reward (settlement compliance points) through playing other content. If both the holmdow and succubus merchants are saying trade is well, you may have already completed the quest, and it just didn't get removed from your quest journal for some reason.
For the Telind quest, each settlement (Vlester, holmdow, and the succubus village) have merchants or shop owners to talk to. You might need to do more content in Vlester and Holmdow to unlock the merchants for the quest, however. If you think the quest is bugged, and can't progress after exhausting other content in these settlements, you can post your save, and I'll see if I can find the problem when I have time to.