That idea is the most plausible explanation for why the gholam was used to kill Fel that I have read. Nice!

I agree that Trollocs would be rather unreliable to leave behind to handle the job of killing Barthanes. Not only that though, it would have been nearly impossible to leave them hidden in Barthanes' mansion w/o being noticed. As for a myrrdraal being left behind to do the murder, did Fain have one at his disposal at this point? I recall that he nailed the one to the door in that town while running from Fal Dara, and he acquired another one later in the Two Rivers, but can't remember if he had control of one by this point in the story that we knew about. (I'm a bit rusty with my WoT)

The reason I suggested Ishamael was because of his ability to locate Rand at will. Finding out that Rand had attended a party at Barthanes' mansion would be fairly easy with his spies and DF connections. Ishy being able to locate Fain is a different story, according to RJ, (since he somehow) exists outside the Pattern, IIRC.