I'm not sure Mac could have completely transubstantiated and still match with what we've seen in the books. When Uriel lent his Grace to Michael in Skin Game and started bleeding, there's no mention of the cut healing, and I think Uriel has to stem the blood.
I'm firmly in the "Mac was an antediluvian Watcher who Fell because he sired at least one child with a human" camp, but you do raise an interesting counterproposal. It does indeed seem strange that Mac was brought to Demonreach. Harry's connection with Mac is pretty damn loose. It'd be like kidnapping Jason Bourne's barber, wouldn't it? Of course he'd
try to get him back, but it's not like Harry's relationship with Mac is extremely close or anything. I've at least never gotten that sense.
Molly, Carlos, Murphy, Susan (pre-death, obviously), Maggie, Michael, Thomas, Butters, Luccio... all of them would make better motivating targets to use against Harry. Mac's on the list, but he's somewhere near the middle at best, in my opinion. And Justine alone would've been plenty to use as a hostage. Sarilla makes sense too, for the reasons you mentioned—good to have the backup vessels at hand, and that's all shown clearly.
So why Mac? There's something to it, I completely agree.
I see two possibilities that can fit. First is your theory: I could be totally wrong, and Mac indeed used his Grace to help build Demonreach. "Raphael" would fit, at least in terms of the name's meaning, which literally means "God heals," (Hebrew origin) so that meshes well with the one supernatural ability Mac's demonstrated on the page. "McAnally" means "Son of the poor man" (Gaelic origin), if anyone's interested.
To continue in this vein, I'll ask an absurd question: what if Demonreach (the genius loci itself/Alfred, not the prison or the Island)
is Mac's old Grace? When Lasciel and Harry had a "baby," it became a spirit of Intellect. Angels themselves are specifically mentioned as possessing Intellectus during the explanation in Turn Coat:
"Intellectus," I said. "Um. It's a mode of existence for a very few rare and powerful supernatural beings—angels have it. I'm willing to bet Mother Winter and Mother Summer have it. For beings with intellectus, all reality exists in one piece, one place, one moment, and they can look at the whole thing." (Turn Coat, page 281 on Nook).
The interesting part to me is that Intellectus is described in two completely different ways in the same paragraph. I underlined and bolded the part that's relevant to me: if it is indeed a "mode of existence" (rather than something an entity possesses), that indicates, to me, that Intellectus is a
state more than it is a capability. Harry's experience with it is one that is shared, not something Harry himself has.
So in this adjacent theory, I think you're mostly right, but I'd posit that the Demonreach entity was born of Mac's Grace, rather than something he lent to Merlin (the way Uriel lent his to Michael). That very well might have been what allowed Merlin to construct the prison across time; because he was working with Grace, which (going by Harry's words) is a
mode of existence that experiences time all at once, all he had to do was form some kind of thaumaturgic connection between the Grace and the physical location. That connection would need to be strong, but not necessarily ultra complicated, which is kinda how Bob describes the runes Harry has him examine, if I recall correctly—the complexity comes from them overlapping across time, I think, not from anything native to the wards themselves.
Possibility number two goes back to Mac being one of the Watchers. The Book of Enoch states that the angels who sired the Nephilim Fell, and were "bound in the valleys of the Earth," until Judgment Day.
Wild theory: what if the prison of Demonreach was the location in which those Fallen angels were bound? I posited in another thread that 8-10k years ago, Chicago was the bottom of a lake, carved out by retreating glaciers, and that Chicago may very well be the valley described Enoch. Demonreach itself is even further below Chicago (Chicago Under Chicago, maybe, the opposite to Chicago Over Chicago depicted in Summer Knight). Mac, a Watcher, sired a Nephilim with a human, and in order to avoid permanent bondage in Demonreach, chose to transubstantiate. His Grace instead was caught and imprisoned there. It's also hypothetically possible in this wild theory that "out" literally means "out of prison," meaning Mac escaped Demonreach, maybe by leaving his Grace behind as ballast (the way Santa stays behind in Cold Days while Harry and the Wild Hunt head to the Island). It's possible there'd be a reaction if Mac is injured so close to his imprisoned Grace, or maybe the "attack" on the prison was somehow using the connection between Mac and his Grace.
Well, anyway, I'm at least glad we can think about this for real rather than throwing out WMGs about what Mac even is. I remember someone claiming (maybe on here, maybe elsewhere; it was many years ago) that Mac was the original Mother Summer, who abdicated. When it's obvious that that title belongs to Mrs. Spunkelcrief.
