I've been out for a while (Darth Real Life), and expect to be busy for a bit longer, but this thread is too interesting not to show it some love (so much so, that I'm writing this a second time after accidentally deleting my post !!@!@#).
This is serious headache-inducing stuff, so I will probably re-read everything again, but for now, a couple of points:
1) Serack pointed this out in the OP, but it bears repeating that while a dipole model is easy to visualize, there is no reason why there may not be more than two ultimate powers. Note that even in the Amberverse, there is arguably a third ultimate power by the end of the first series (though technically two of the powers are on the same side. Kinda).
2) I really liked KG's True Name == IP address theory. Consider how we are told how a mortal's True Name changes slowly by the decisions they make (and see Harry description of calling Elaine in WN), and how it parallels the creation of new universes by Free Will. Basically, the difference between the same individual in two difference universes is the Choices they made. Thus, their Names are ever-so-slightly different, and saying their name in a very particular manner will choose one or the other.
In fact, this may be how MM Harry is summoning his doubles. He basically summons himself but says his Name just a random tiny difference, so the spell summons one of his "nearby" clones.
3) I think Griffyn's point about Uriel giving up his Grace is important. It seems inconceivable to me that an Ultimate Power would risk so much just for a minor win in a single universe. If Michael had failed, and Uriel had Fallen, would he have Fallen across all universes? This seems a little crazy. If, on the other hand Uri himself is a local manifestation, then this is more plausible.
Note that it is very possible that Uri is common to multiple universes, just not *every* universe. Consider worlds in which the Arahamic religions arose in such a way that "Falling" worked with a diffent mechanic (heck, I don't think Judaism/Islam has the notion of a "Fallen Angel" -- this is only really a Christian thing). I don't think Uri Falling in our universe could affect Uri's manifestion in such a reality.
From wikipedia:
No angel is able to disobey God due to the way God created angels. For this reason, Islam does not teach that Iblīs or Shayṭan (the Devil or Satan) was a fallen angel, rather he was one of the jinn.
4) Further down this line of thought, I think we need to consider the entire WG "pantheon" (WG/Uri/Luci/etc.) as manifestation of a single entity -- no different than Summer/Winter.
Actually, here is a
nasty cool twist (heheheheh):
We know that Mab has the job of protecting the Outer Gates while Titania protects humanity from Mab. In similar vein, perhaps it is Hell/Luci's job to protect Humanity from XXXXXXX (Nemesis?) while it is Heaven's job to protect humanity's Free Will from Hell?
5) I really like the Edna/cutting off finger analogy. It fits very nicely with the narrative as I see it. It also works well with the whole Oblivion thing. Basically, the whole war is about changing
Humanity so that certain less-savory aspects cannot manifest. It would still be worth exploring the notion of imprisoning such power (Demonreach/Coins). If we hold that "the Fingers just regrow", then it seems hard to explain why putting such powers in a position where they can't affect the world is fundementally different from "Oblivion". It's also interesting to think of this in context of Halloween. On the one hand, it seems that Immortals can only be changed at specfic conjunctions. On the other hand, the power of mortal belief can change the mantle (apparently) continuesly. This may be accessing two different parts of the manifestation...
I'd really recommend reading C.S. Friedman's
Coldfire Trilogy. One of the ideas behind her world building is a very logical (and coherently described) system where human belief actually shapes how the demons/elemental forces of the world work (it also has similarities to Furycrafting for Alera fans).
6) Another thing I thought I saw mentioned (by Serack?), but the whole "Conservation of History" is interesting. If you go back in time and change something, are you changing the same event in all the universes that spawned since then? If yes, then this possibly explains how it gets harder to change something the farther back/bigger it is. You are not only moving the tiller on your universe, but on countless others, making for a larger multimetawhatever mass to shift.
7) Finally, it seems (WoJ) that all these universes are somehow accessible through the Nevernever. If this means that there is only a single Nevernever -- essentially a vast ocean with isolated islands of "reality" floating around in it. I feel this could be an important point in the grand scheme of things, though I am unsure yet how.