Giving validity to your claim that only leaves linkz as the only possible third party. Yes, but do you really think having a possible gravemind with a fakeclaim of master chief? I don't think it would extend past the minor characters, as was the case in the past, with the members of noble team and especially captain Keyes I am thinking, which is pretty much a sub character which dies off halfway through the first game.
I am honestly thinking its linkz with another cult member on the loose, which he cold have converted either night 1 or 2, seeing as you claim would also clear radish as being unaffected by the attempted possession, which is weird. Either way, even if we do get the recruiter that will still leave for a tough ending as there is a chance that the people you've scanned before, or even you, or even spurge could have been converted since.
Disconnect and self destruct, one bullet a time.