With respect to Sidhe vs Fey, it could also be a matter of feudal courtesy. Since the fey were driven underground into the Sidhe barrows, it is possible that the Sidhe are/were the original lords of their respective barrows. In midevil Europe it was common for the land to be named after the local lord and vice versa. This could mean that Lieanshide would translate into modern English as "Liean, lord of the barrow". This would also help to explain why the Sidhe hate to be called Fey; it would be like walking up to the duke of York and saying, "hey mister! You got the time?" 

Insofar as the Sidhe once being mortal, we know the Fey are incapable of changing their natures (at least very quickly), so what if a number of human wizards agreed to lead the Fey into the barrows and keep them there? They may have started as their jailers but were slowly changed by their exposure to the nevernever into fey themselves. This would explain why the Sidhe are known for always followings rules and obeying agreements while the rest of the Fey only do so out of fear of the courts. The Redcap would happily have gutted every mortal he came across were it not for the threat of Mab, and I seem to remember Harry being warned that the queen is the only one who cannot harm a mortal who doesn't associate with the Fey, but the members of the courts choose not to harm mortals out of respect/fear for the queens.

My apologies if this has already been discussed, I haven't worked my way through all seven pages yet.

The arguement is that Mac was a god.  Remember, Mac says "I'm out" and then later Odin reveals that it is possible to give up a mantle on certain confluxes. 

The story I see goes like this: Ragnarok occurs, the guard changes and Heimdal finds himself out of a job. Unemployed, depressed, and Norwegian, Heimdal decides to deal with his midlife crisis by getting completely blotto for a few centuries/millenia. When he finally sobers up he decides to get his act together and do something productive in his retirement; forsaking his mantle and opening a bar in Chicago (possibly after power watching every episode of Cheers.) And if his chosen venue should happen to allow him to keep an eye on a certain starborn wizard, well...

I want to see Harry team up with the Flying Spagetti Monster. 

I think Michael Cane would make a great Nicodemus.  He is the perfect counterpoint to the ravenous monsters and psychotics that make up the rest of the Denarians, and being cast as Nick would give him opportunities to give Harry one of his patented morality lectures, only this time it ends with "... and that, my dear boy, is why they all must die."



Edited for spoilerieness

I went looking and found a few theories that are due to expire in a month or less:

http://www.jimbutcheronline.com/bb/index.php/topic,26893.0/topicseen.html
http://www.jimbutcheronline.com/bb/index.php/topic,26905.0.html
http://www.jimbutcheronline.com/bb/index.php/topic,26888.0.html
http://www.jimbutcheronline.com/bb/index.php/topic,26976.0.html
http://www.jimbutcheronline.com/bb/index.php/topic,27034.0.html
http://www.jimbutcheronline.com/bb/index.php/topic,27096.0.html
http://www.jimbutcheronline.com/bb/index.php/topic,26872.0.html

I don't think there are any duplicates with what has already been posted.  I just went looking for any old threads I could find that the OP postulated a theory with any sort of references to the books for evidence.

I leave it to the curators to decide if any of them have merit.

^It would explain the shotgun.

Has anyone considered wether or not Harry met Thomas in his apartment?

Suppose another wizard finessed his (or her) way in past the wards once the house was emptied and started fixing LC.  (S)he takes a couple of hours to deconstruct and repair the spell matrix and starts packing up to leave.  While heading for the exit they hear Harry opening his door and cast a quick glamour to look like Thomas.  Now, this mystery mage would be waiting for the other shoe to drop, not knowing if the glamour would hold.  Since a wizard trying to secretly fix LC and help Harry would not want to risk leaving any magical clues (or break the first law) (s)he levels a shotgun as a last line of defense in the event that Harry notices the deception and attacks.

If we account for time travel it could have been Harry talking to himself disguised as Thomas while he was trying to escape from his own appartment after fixing his own broken spell.  (has your head exploded yet?)