Facts:
...
True Names change in humans. 

Do we know how often, or easily, the True Names change in humans, though?  I  don't recall this coming up in the canon stories; is there some nice clear WoJ?

Jim has been clear that two people cannot re-Soulgaze... WoJ provides an exceptional case: if one of them has undergone a wrenching, soul-redefining change (something exceedingly rare... Harry, for example, withstood a Fallen Shadow, and even converted her, has became the Winter Knight, with the mark of Winter deep inside his spirit; he has died (mostly (y'all know what I mean)). And yet none of these, singly or collectively, has changed him enough:  everyone he has previously Soulgazed is still safe to look into his eyes).  I suspect a re-Naming might take a similar level of change.

... I wouldn't be surprised if Michael knew Harry was coming back. I'm certain he is the type to have prayed about decisions as to Maggie's paperwork. (Whether to do it, what choices to make). And Michael often gets clear and direct answers.

If he knew, he would have chosen Dresden to help push Harry into accepting his responsibilities. Charity even more so. 

Although, it's worth noting that the sorts of  Answers  that Michael gets back are not always the most-obvious/expected ones.  Maybe the whole Rodriguez/Mendoza/Dresden muddle would actually benefit Maggie to be further muddled (in protective, hard-to-track-down ways) by giving her the Carpenter name.  They wouldn't hesitate to extend their protection that way.

Nor does anyone else at this point because the nitty gritty of just what Maggie's true name really is, at the moment is pure speculation.

I wouldn't call it pure speculation.
Jim has given us plenty of hints about True Naming, and plenty of facts about little Maggie's possible names.

I'd call it "educated guesswork," and made my best guess above.  I won't be utterly shocked to be proven wrong, though!

... It was the surprise at that point that gave Harry the advantage against Thorny at the Shedd ...

I'm reminded of story I heard about a very-senior jiu-jitsu sensei who was having a Very Bad Day, he was angry about I-don't-remember-what.

Somebody unaware of his mental state managed to walk up behind him, clap their hands over his eyes meaning to say "guess who" but before the first syllable was out of their mouth they were airborne in a the kind of throw that ends in a full-body slam onto the floor with both hands tangled & unable to slap a breakfall at all.

Also reminded of Harry triggering one of Eb's defense-contingencies.

When you "get surprise" on someone, best make sure they aren't much, much better than you.

I absolutely do not believe utterly-new-to-Soulfire Harry could have so utterly manhandled any Angelic being... it is beyond "implausible."

(fwiw:  the sensei mentioned above (once their "foe" was in front of them instead of behind, i.e. could *see* them) was able to alter the throw to be setting-them-down-upon-their-feet)

... It's logical that Namshiel would want a fortress like Merlin's Castle to work out of, he may have wanted it for a long time. He finally has a host, i.e. Marcone, who actually had the financial backing and means of making it happen.  Reread the bit in Twelve Months where Harry does the "Wizard of Oz" trick using the leylines under the castle as a power source, when he warns the vanilla demonstrators.  That's why Namshiel wanted that particular location, the leylines that just happen to cross where Harry's basement lab was located.  Oh another reason, remember Lea's garden is also located at that particular location.  Result there is no reason to conclude that Namshiel is working or attempting to work on the same side as Harry based on the evidence presented.

Consider your accounting of Thorned Namshiel's motivations and desires here.  He "finally has a host" who can get him the castle/base/HQ/lair/whatever that he has long dreamed of.

Consider the scene where Harry demands the castle.  Does Namshiel just allow Marcone to give up that long-sought prize??!?  Once again, this bursts the bonds of credulity.

This was all maneuvering to get the castle into the Well-Warden's possession.

... After Twelve Months, I have a creeping feeling that the conclusion of the Dresden Files will reveal that the entire series was 'True Love Conquers the Outsiders.' 

Doubtful.

"True Love" conquers the Raith-style lust-Hunger.
But it takes True Bravery to conquer a Skavis, True Hope to conquer a Malvora.

Those are specifically Hunger-Demon Outsiders, however:  not "Outsiders" in general; we've never seen any of that being particularly potent against other Outsiders.

I think the key won't be "True Love" but mortal "Free Will."

... Time was running out, Harry's only recourse was to go to Mab.  It worked out playing by the old rules, not by human law, the mercy was Thomas had to give up the child to pay for the life he chose to take.

Yeah, it was a very, very old-school / dark-faerie sort of resolution to the conundrum.

I imagine Mab was absolutely delighted, on a personal basis.  More fun than she'd had in centuries, likely!

I suspect Namshiel may be on a redemption arc, un-Falling.
Or at least is being an ally-of-convenience against the Outsiders, working with Uriel (or other unFallen angels).

1. If he was such a badass angel-wizard, how could Harry have beaten him back at first meeting in the Shedd?  Soulfire was brand-new to Harry, but well-known to Thorny; angels are essentially pure soul, so Thorny has *WAY* more magic muscle to throw around in that particular arm-wrestling match, and it's his "native tongue" so to speak, utterly familiar to him for millions of years.

Maybe ... Thorned Namshiel threw that fight?

2. It's just way, way too convenient for Merlin's Castle to have been imported from Europe to the shores of Lake Michigan and set up precisely atop Harry's old laboratory, just in time for the Warden of Merlin's Well to return, and take the Castle off Marcone's hands.

I can only conclude that Thorned Namshiel wanted Dresden in possession of the Castle!

Taken together, these sure make it look like Namshiel is working on the same side as Harry...

...  Was he hanging out with Butters and Father Forthill watching the boxing match, and heard about it at the same time? ...

I suspect he may have been, actually.
Because it seemed an awful lot like Forthill and Aaronson arrived in coordination with Ramirez -- they knew he was veiled, part of their approach was to help his stealth.

I think Carlos was raised Catholic -- he still curses with Dios! and other religious curse-words, not Harry's supernatural "Stars & Stones!" -- and maybe he has a connection with Forthill by way of a similar priest back in LA, likely another member of the Ordo Malleus (this might give Carlos some "cover" with the White Council, interfacing with an ally is a legit non-Dresden reason to be in Chicago).