The passage of time during which viewers don't have the chance to see what is transpiring between Tieran and his confederates does leave an opening for interpretation and uncertainty. Though, again, from what we are allowed to see of their interactions, would seem to suggest the uncontested ascendancy of a shrewd, canny, and supremely confident operator like Tieran, playing out his designs, rather than a partial takeover in which both personalities are vying for control and are in conflict.
The scene that you cited some dialogue from between Tieran and Adin is interesting given some thought. Adin is conducting an examination to verify that the impact of the transfer is fully in place. It can leave open to question, why such a check wasn't done on board Voyager at Adin's earliest opportunity, is it an indication that such a test had been earlier done, perhaps with less than conclusive results, or might it be a simple follow-up check that Adin conducted with all of Tieran's hosts? Again, maybe an indication of the special nature of this transfer, as you suggest.
I do agree that the willingness and realization that a rupture with Neelix was what Kes truly desired, and that she didn't fight being led to the action, if she could've, by Tieran, as I believe was the case, also simply served to expedite the time when the decision would have been ultimately followed through on. Whether she had conviction to articulate it on her own at that time, it seems clear that it was, in fact, what she wanted, I think it is telling to notice that when the action is over, Kes seems to to not want to face Neelix directly and it's not him with whom she is meeting with back on Voyager, it's Tuvok.
