[member="sabrina"]
Unfortunately, facts have little place in the topic, beyond what has happened in the past (we create our own facts in this setting). This is largely an opinion based discussion until a choice is made by the admins of the group, or the writers of the group can come to a consensus. Facts go as far as to tell us what has happened in specific instances in the past, not what should happen right now. Why I think Harley can't be possessed by the Dark Lord? Because I don't think Harley has been involved in enough goings ons in the One Sith for the Dark Lord to enter her body. I think it would take a large load of devotion to the DLoS in order to contain his body as well as a formidable level of Force knowledge/control. Sure, your character may have that, but I don't think the Dark Lord would pick any old candidate purely off of proximity. He's the dark lord of the sith, he's not falling into desperation and would be fine floating about to find the best candidate (of which, I think [member="Silara Vantai"] is if we were to go purely off of devotion and faith in the power of the Lord...and would also help move towards development of the sithspawn).
She specializes in beast control and battle meditation, both of which are powers that I think would go a long way towards maintaining herself while hosting the Dark Lord within (as they involve maintaining control of oneself while constantly influencing others). It's not necessarily that Harley can't do it, I just think that there are better candidates based on development. On top of that, I think it would be far too destructive to Harley for the Dark Lord of the Sith to consider possessing her (because of her current lack of development). And I think, as he is likely looking for a place to stay for a while, that would be something he would consider before mounting someone, as it were.
These are my opinions and purely that, opinions. If it were Darth Banshee and she were still part of the faction, we would be having an entirely different conversation. Along with this, I'm not entirely sure we even need to go the route of possession. I think the Dark Lord of the Sith is powerful enough to roam free for a bit, take his form when it's ready for him.
Those are my opinions on the matter.