Not a lick other than what I cited. Mostly just me juggling pieces of the books and tossing random interesting thoughts into the fray. Using the phrase "compatibility issue" is kind of strange, as is the fact that it was from a choice she made as a mortal.
Then there is this.
tension-tightened faces. “Old woman,” Corb taunted. “I remember you as a bawling brat. I remember your pimply face when you rode with the Conqueror. I remember how you wept when Merlin cast you out.” Mab’s face . . . . . . twisted into naked, ugly, absolute rage. Her body became so rigid, so immobile, that it could not possibly have belonged to a living thing. “Tell me,” Corb purred. “If he was yet among the living, do you think he would still love you? Would he be so proud of what you’ve become?”
Butcher, Jim. Peace Talks (Dresden Files) (pp. 278-279). Penguin Publishing Group. Kindle Edition.
So Mab and Merlin were a thing. Yet he cast her out, for what? Having a child with a succubus?
Cold Days pretty much lays out the blueprint for time travel as Jim apparently sees it. Branching time and the multiverse. He has Odin imply that this has all happened before. It may be a Groundhog Day type of loop or something like it and in a WOJ Jim says he's going to use the time travel book to fix the continuity issues.
The castle is a perfect takeoff point for time travel through portals. If it is Merlin's castle then the portal affinity would be there is you assume that Harry will go back and become Merlin. The set pieces would be Eb's death and Harry coming into possession of Merlin's diaries. The reveal of the true nature of Bonea.
A short ride through my nightmare.
At a moment in the last book when all hope appears to be lost Harry escapes through a portal to the past sent by Mab or Bonea. The reveals in Merlin's diaries and the aid of Bonea will be the keys to what he has to do. Bonea will be the guide to creating Demonreach in prehistory along with Odin and in doing so she will hatch the plan to eliminate the Old Gods and become the Archive. She will gift to Harry her gift of languages. He will sleep in Demonreach as he goes back to the future appearing at the key points in the Story to do whatever Jim needs him to do. Harry builds the castle on Demonreach Island and later moves it to await Marcone.
He fixes LC in Proven Guilty, warns himself that the Hobs will attack in Small Favor and are agents of Winter and is probably the genesis of Mab's snow scene showing Marcone's kidnapping. Jim must have been on a Calvin and Hobbes kick at the writing.
Once Harry Prime is gone Merlin will appear. Mab will be revealed as Harry's true love and will meet the fate of his other loves when she is back stabbed by Molly who will complete her trip to evil. And so on.