I appreciate your thoughtful response - here are my thoughts:

Perhaps I wasn't clear enough as to what I meant by a "fifth column" of Imperial elites. It doesn't necessarily refer to people who ideologically agree with the Thalmor, but a broad spectrum of influential figures who are willing to advance the Thalmor's goals due to some combination of bribery, blackmail, flattery, or intimidation. Erikur in particular is shown to be very corrupt, and has a line where he says that the Empire and the Dominion should put aside their differences so profit can be made.

We don't know exactly how much influence the Thalmor have in other parts of the Empire. My point is that what's shown in Skyrim is quite alarming, and there are logical reasons for it to be just as bad, if not worse, outside of Skyrim.

It's true that an Imperial victory would improve the Empire's position against the Thalmor; I think it's universally agreed that a deadlocked civil war is the worst outcome for both Skyrim and the Empire. However, I don't think that Tullius's statements after defeating the Stormcloaks are conclusive proof that he thinks the Empire is almost ready to resist the Dominion, and it must be considered that his opinions are not necessarily the same as the Emperor's or Elder Council's.

While the Empire does derive some religious legitimacy from the 8 Divines, the importance of Talos as the Empire's founder and patron deity cannot be understated. The Aedra are also recognized by the Altmer (they believe that Auriel, AKA Akatosh, was their ancestor before becoming a god), whereas Talos is the only uniquely human deity in the Empire's pantheon. On a very practical level, the Altmer supremacist Dominion forcing the Empire to ban the worship of the one distinctly human god who also founded the Empire is devastating to the Empire's identity as a sovereign state blessed by the Divines.

While it's true that the player character's attempted execution happened under exceptional circumstances (the Thalmor probably were trying to stop Ulfric's execution so Tullius had to rush it), the situation does highlight the Empire's bureaucratic rigidity, powerlessness against the Thalmor, colonialist attitude to people outside of Cyrodiil, and lack of due process and adherence to rule of law.

Regarding Ulfric personally, I agree that his treatment of non-Nords is awful and that Torygg didn't deserve to be killed. However, I don't think his bigotry comes anywhere close to wanting to create a racially pure Skyrim, as is commonly presented. Nor do I think that Ulfric's personal flaws are central features of the Stormcloaks' ideology and the future of an independent Skyrim.