The phrase is unreliable narrator. Jim Butcher likes to keep you guessing and I think Bob is one of the tools he uses to do that. Take the idea of killing an immortal. It isn't technically true. The mantle doesn't die, it simply moves. And that is the immortal part, not the person who wears it. And in Ghost story Mab is telling Demonreach Harry is her to mold as she sees fit. And then there is this.
And a voice--a very calm, very gentle, very rational voice whispered in my ear. "Lies. Mab cannot change who you are."
Yet later in Cold Days Bob tells Harry he is doomed to become like Slate. Draw your own conclusion.
Well, Merlin must have gone some distance into the past to lay the spell for the prison in five different times. But the stone buildings may not have been created at the earliest point he visited the island.
For one thing, even if he quarried and assembled the stones by magic, the fishing town that occupied the island somewhere around 1900 probably would have noticed something out of place if the architecture of the lighthouse was characteristically pre-medieval European from Merlin's native time, and showed obvious centuries worth of wear. Chances are Merlin convinced some settlers to build the structures on one of his forward excursions through time, and dropped his own enchantments on top.
The obvious question is, why would he do that? Consider that magically he built the caves that lie beneath as well as the cells.
Tonight is my wacky doodle night for weird theories. The trilogy titles are,
Hells Bells
Stars and Stones
Empty Night
Basic premise. The trilogy will be when the well gets built and populated. One of the weird things about Merlin is that he knows when things happen. At some point in time he goes into the Library of Alexandria to save some of the books. He helps found the Church in Rome. Almost as if he knew these events would take place.
Well maybe he did, maybe he went into the past to save the future. He could be Harry. Vadderung knows something, because he uses the term banefire when talking about the fail safe. So far in the book there have been two types of magical fire in the books, hell fire and soul fire. No one prior to Cold Days has used it or mentioned banefire. And fire is Harry's thing.
Jim has revealed the Oblivion War. If it wasn't going to come up later why bring it up at all? When that battle is over all of the beings in the well will be sealed out of reality, unable to effect events thereafter. Now Jim says the Oblivion War will never come up in the books, and that could well be misinformation. It could come up in the trilogy. The well, the archive, and the other mysterious constructs came from somewhere. And were the creatures to escape from the well, then the Oblivion War would be lost. And last, for the Oblivion War to be possible, someone had to confine the Old Ones and their kind. So that they couldn't act.
A couple of other things. Ivy is in her way very similar to Bob, with a human host. It is at least in the realm of possible outcomes, that Bonea is the foundation for the archive. Lash had more knowledge of reality than even Bob. And her child would know some portion of what she knew. And would be a very powerful spirit.
I don't know that I take this seriously, but it is a rough outline of how I might end it.