Hiya! Do you have a theory about the Hooded Man? Although Hallis Mollen seems like the most viable option, I quite like the idea of Harwin being the BWB eyes in Winterfell like Tom is the eyes in Riverrun.
To be fair, we don’t know that Hallis Mollen isn’t with the Brotherhood. He disappeared somewhere between Riverrun and Moat Cailin, which is their turf. There’s 3-4 months between his leaving Riverrun and Arya’s break from the Brotherhood, then another 8-9 months until Theon encounters the hooded man. That’s a believable timeline for him to go north a ways (leaving Ned’s bones at Greywater Watch, maybe? Since Howland Reed gets sucked into all the rest of the Stark drama), meet the Brotherhood, and be sent to Winterfell. I don‘t have much of an argument to say that is the case, just that I don’t see reason to rule it out.
There’s less time for Harwin to go north, since he’s present when the BWB encounters Brienne and doesn’t seem to have any interest in leaving before they’ve dealt with Jaime. But as you say, there’s just as much reason for them to want an agent there as in Riverrun.
I like the Theon Durden theory. It doesn’t even matter so much whether it’s factually true (probably not) but that it’s possible. Theon’s chapters in ADWD are nightmarishly surreal, with his sense of reality so unstable that we don’t know what’s real either. Whether he’s talking to himself or not, the externalization of the character’s internal recriminations and his halfhearted defenses of himself are a part of his journey back to Theon.