#5 I disagree with. While it would be neat to join the Thalmor, it would only work in one specific scenario. The Thalmor also aren't the baddest of the bad. They took just as heavy losses as the Empire in the war, the White Gold Concordat wasn't just to get them into the Empire so that they could wipe out Talos worship, it was also because their forces were very much decimated as much as the Empires was.
You can't really make the argument that the Thalmor can "hold onto what they take". Example? Hammerfell. As part of the WGC, the Empire was supposed to hand over half of Hammerfell to the Thalmor because it would be a strategic border for the Dominion. The Redguards kept fighting, refusing to honor the WGC. The Empire told them to quit fighting the Thalmor, Hammerfell said "Screw you, we're not giving up our land."
The Thalmor threatened the war to start again if the Empire didn't do something (stupid bluff by a very weakened Thalmor), so the Empire kicked Hammerfell out of the Empire. Hammerfell is it's own Sovereign Kingdom again. So the Redguards
kept fighting the Thalmor and soundly kicked them out of all of Hammerfell.
The Thalmor got their asses kicked by Hammerfell and lost all of the land that they tried to capture there. If Hammerfell can whoop that ass, then so can the Nords of Skyrim. In fact, it was Nords and Redguards on the front lines of the war that caused the Thalmor such heavy losses.
All of what I've just said is in the Lore, as well as easily obtainable in in-game books (you
are reading those, right?

).
So to join the Thalmor may not be as grand of an idea as everyone may think. The only reason that they're in Skyrim is to wipe out Talos worship, but also to keep the Civil War going. As long as their is a Civil War, then there's less Nords to kick their ass a second time. And also, the Thalmor actually fear Skyrim breaking away from the Empire. If Skyrim goes secedes from the Empire, and allies with Hammerfell, the combined might of a Redguard/Nord army is really more than the Aldmeri Dominion would be able to handle.
Just my views on this new suggestion.