Personally, I'm intrigued at the idea of the Thalmor still having a presence in Hammerfell. Supposedly, the Aldmeri Dominion forces withdrew from Hammerfell after the signing of the Second Treaty of Stros M'Kai, though, admittedly, a lot could have happened in between the writing of "The Great War" and the events of The Elder Scrolls: V - Skyrim. The prospect that Kematu is lying and actually working for the Aldmeri Dominion is, while somewhat less likely, still very interesting. If the Thalmor do still have some presence in Hammerfell, perhaps they control an enclave of some sort, perhaps even Stros M'Kai. I however find it more likely that the Thalmor have agents spread throughout Hammerfell, partaking in assassinations, intelligence gathering, sabotage and the recovery of precious artifacts of the Yokudan or Dwarven variety. Really anything to combat the reemergent Ansei. Plus, this way, the Thalmor could direct role in the Satakals becoming major antagonists, the Great War poisoning their ideology and manipulating them into pursuing a religious uprising against the Forebears and the Crowns, not understanding the true scope of their power, nor their plans, to resurrect the Celestial Sep, the Serpent to remake Nirn.
As for that "Summersets" easter egg, it seems rather convincing. While we can brush off most references to other provinces in Skyrim as just world-building, a province being name-dropped like that in Fallout can only be either a teaser or a somewhat random though admittedly clever easter egg. Imagine if some Breton mentioned a "commonwealth" in High Rock or something of that caliber. Basically, if Alinor is announced as the setting in a few years time, I won't be surprised.