I'm going to put on my tinfoil hat and join the "guess the reincarnation game".
Let’s start with the fan favourite stalker, Adrian. Like everyone else and their mothers, I suspect him to be Lancelot or Mordred, since both of them would explain his self-deprecating tendencies and visceral guilt. But, imho, the mysterious woman in his POV is one of the Lady of the Lake, probably Nimue, since Vivian is otherwise occupied with the repercussions of a certain demon’s actions.... From one of the Q&As, we know that Vivian, as the Lady of the Lake, would be capable of recognizing Lancelot’s reincarnation, so it is possible that even a deposed Nimue might have the same ability. This would also explain his distrust of fae and possibly clairvoyants, if Nimue is also one of them. Nimue is also one of my favourite possible parents for Changeling!MC, since Adrian seems to distrust Changeling!MC’s mother and Merlin suggest that the homicidal inhuman tendencies came from the mother and not the father of this MC. But more importantly, the absolute drama of Nimue being the mother of a Guinevere’s reincarnation that is romancing Adrian du Lac.
Now with Percy, I think he might be connected to Corbenic and the Grail guardians, but who specifically idk, maybe Elaine or Percival- although Percival is possibly a red herring because it’s too obvious. Also, the conditions to be Percy’s soulmate make me doubt he might be the saboteur.
For Cassandra, I know many suspect her to be a fae infiltrator cough Morgana cough, but I actually think that to be a misdirection. I suspect that Cass is Morgause’s reincarnation. There is some sort of irony in Morgause being confused with Morgana; what with the amalgamation of the two in modern culture? Morgause is a descendant of Ninianne, so she also has a connection to Avalon, and is a highly skilled sorceress whose magic is described as more water than fire in one of the Q&As. And about water, the concealing spell, used in chapter 2 to hide the harbingers, is described by Changeling!MC as a passing ocean weave... What if Cass!Morgause has already awakened her powers and cast the spell herself? And if she were a fea, Changeling!MC and Merlin would know immediately -Merlin can tell that MC is a changeling even with their state of the art conceiling mambo jumbo.
Gwen is the more likely “broken lamb with the heart of a wolf” and saboteur, and I tend to agree with Percy’s Guinevak theory. But, what if she isn’t the saboteur? I suspect she might be Isolde’s reincarnation. Gwen claims to be a queen or a princess in her dream/memory, and she loves romance and courtly love. Everyone in the group is hoping for her to be a powerful magic user because of her low body stats, but what if she is the healer of the group, and the best option for this role is Isolde, a known healer in Arthurian lore. And even if Tristan is truly dead there is still Cador around... Good luck Gwen with the repercussions of curtly love.
Lorelai, idk why but the whole “I hate knights and their chivalry” reminds me of Mordred. There’s something dark going on with her , especially with that dream Arthur witnessed, that also imho points to Mordred. If Adrian is Mordred, then she might be Lancelot.
Broderick John Doe, I don’t have theories other than his clearly fake name and identity and possible saboteur. But if I have to choose a reincarnation for him, I would go with Kay.
And here's the call to action for more theorizing in the comments!