--As to the tuning, the main character would have to relearn how to make certain chords (to "work around" the out-of-tune strings.)...One: is that feasible?

That really depends on how it's out of tune.  If a guitar's out of tune, it normally means in relation to the other strings.  Do you play an instrument?  It means one string will, when open (without a finger on it), be somewhere between E and F, or between C and C#.  The other strings will be out of tune in different ways-- one will be halfway between the notes, one will be 1% off one note.  Some of the top strings may be in tune; the bottom (thinner) strings normally stretch faster.  Frets are spaced to automatically give you a half-step up, so the same relative out-of-tuneness will be kept no matter where you play on the string.  Now, if it's a fretless guitar, or if he can remove the frets, he should be able to get around it just moving his fingers slightly, but he wouldn't need completely new fingering.  If it's only slightly out of tune, that would be close enough to standard fretting for him not to have problems when playing on a new guitar.

You can tune a guitar to something other than the standard 12-tone for a specific song, and if it's out of tune as in specifically set to that tuning to play an experimental or non-western song, he'll be able to play any song which uses that tuning, but he's not going to be able to play your average song with an odd tuning.  And it'll be just about impossible to play with anyone else without prior notice, and even then you're somewhat limited in your choice of instruments.

But how does one fake the USPS-issued postmark, without a DeLorean and a flux capacitor??

You don't.  You fake the seal.  You never seal it in the first place.  Or you seal it with a paperclip.  Or you carefully open the envelope and redo the glue.  All of which probably adds up to why there are no cases where someone's successfully won with this method.

Tell that to the family of the late Art Buchwald and ask his heirs about "Coming To America" with Eddie Murphy...

Quoting Wikipedia:
"Buchwald was also known for the Buchwald v. Paramount lawsuit, which he and partner Alain Bernheim filed against Paramount Pictures in 1988 in a controversy over the Eddie Murphy film Coming to America; Buchwald claimed Paramount had stolen his script treatment. He won, was awarded damages, and then accepted a settlement from Paramount. The case was the subject of a 1992 book, Fatal Subtraction: The Inside Story of Buchwald V. Paramount by Pierce O'Donnell and Dennis McDougal."

Just read the article on the case.  Paramount optioned a treatment, didn't pick up the option, and then made an ostensibly different movie using an outline remarkably similar to the treatment.  That's more than a story idea, that hits original work of authorship territory.

The line when it comes to ideas is somewhat fuzzy, but for example, one could potentially write a story about a wizard, a member of a wizard council which is at war with vampires, without necessarily infringing on JB's intellectual property.  Or a story about a wizard-PI.  Two authors can write completely different stories based on the same ideas.  That's why section 102b of the Copyright Act says, "In no case does copyright protection for an original work of authorship extend to any idea..."  Of course, there's protection once you get beyond the idea stage.  And depending on the sum of the work, how many other ideas in the work are similar, how many new ideas there are, how many other similarities there are between characters and situations, with the structure of the story, the second author could run into trouble.

but the POV I don't get.

Figure out a structure for your POVs.  Mostly you don't want to shift POVs from one sentence to another.  One of the simple ways to keep them separate is to have chapter breaks or
page breaks.  At least make sure you start a new paragraph when switching POVs.  Finally, be careful if you do rapidly alternate paragraphs between the POVs of characters A, B, and C: ABCBCBACABC.  To a point you can do this in the omniscient POV, and it actually happens in some older books (though maybe not quite that fast).  Try reading most anything published in the 70's/80's and you'll eventually run across it.  The real point is that you don't want to confuse the reader about who's doing/saying what, who knows what, etc.

"Graffiti was all over the whitewashed walls"

I've heard that forms of "to be" are weak verbs.  "Covered," I guess you didn't use it because you use it right after, but maybe you could use it if grime "coated?"  There were a couple other times I saw linking or passive verbs, but that's one that jumped right out, probably because it was at the beginning.

"We got to the right station and quickly took place near the entrance area."

Until I read this, I wasn't completely sure if the characters had just gotten off an elevated train or if they were walking around a train station, a random building, a basement or half-story where you could hear a train, or a subway.  The wind whispering in the guy's hair after he walked down the stairs threw me a little more.  Saying they were walking down "the subway stairs" might be a quick fix, or it might be unnecessary.  This may not be a problem for most readers, I don't know, and it's not too hard to figure out, but I thought I'd point it out.

Not bad.

If you're playing around in what's recognizably Butcher's universe, then you're violating copyright.  On the plus side, as long as you aren't an idiot and don't try publishing your fan fiction, you'll be more likely to get a cease and desist notice than get sued if you get caught.  C&D should be good enough to demonstrate that the author's protecting his/her intellectual property.  If you ignored that, they might take further action.

If you want to be a writer, as in eventually get published, there are a lot of authors who say it's better creatively to just make up your own universe.  If all you want is to write fanfic for your own amusement, well, just don't post it here.  Authors have to defend their copyright to make sure other people don't start selling, for instance, new Harry Dresden books, but writing stuff in your notebook shouldn't cause problems in that regard.

You try to write a novel in a month.  Specifically November.

Doesn't work if it's Excalibur, though; Annales Cambriae records Arthur's reign as 519 - 537 [ Badon to Camlann ]. Though Gildas records Badon as 570, the year of Gildas' own birth; there is some flex there, but not five centuries.

Wait - do the Annales Cambriae actually refer to Arthur as a king?