The "divided loyalties with Winter" angle isn't going to carry much traction with Rashid, that's for sure.
Of course, you don't really need a powerhouse to cap Harry. You'd just need someone who can veil well enough he wouldn't see it coming to wait outside his door and shoot him in the head. Carlos could get that done if he thought it was genuinely right, although I'm not sure he's that far gone.
Langtry could also contract with a supernatural mercenary like Kincaid or Grey - I'm not sure either of them would take the job, but there are probably a few others similarly dangerous. If Langtry would stoop to dealing with him, Shagnasty could probably still take Harry on the mainland.
Good points. A couple additional thoughts.
My point about Rashid was not an issue with loyalty to Winter. I don't think Rashid has loyalty to Winter. The problem is that Rashid's job may be one of the most important in Creation right now. If Rashid were killed or incapacitated, then the Gates could fall.
It seems possible that the White Council wants someone else to do their work for them. That would be a very Langtry thing to do. Hiring a mercenary to do it could be possible. But, how many qualified supernatural mercenaries are available? That would be willing to risk Mab's displeasure. That would be willing to face the Winter Knight? Mercenaries by definition take on risky, not potentially suicidal jobs.
Grey could easily do it, that is true. But would he? And, would the White Council ask him? He's probably known to them as having worked with Harry at least two times now. They would probably scratch Grey off the list.
True, Harry was in some risk in the immediate aftermath of the White Council's decision -- because he had been beaten to an inch of his life, expended vast amounts of energy and everything was in chaos. In a couple months, where Harry has rested up, replenished his gear, established Castle Dresden with defenses, the Winter Knight and Consort of the White Court of Vampires is a certifiable nightmare.