Umm ... have you read what the Thalmor said about him? I don't recall anything about Tullius.
Have you read the dossier? It essentially says that the war is their asset, not Ulfric himself. As such, Tullius is as much an asset to the Thalmor as Ulfric is for not walking out of Skyrim and letting the province have its independence.
Do remember that it says that a Stormcloak victory is just as undesirable to them.
The author shows more signs of being sympathetic to the Forsworn than just making Imperial propoganda. Yes it's biased but I'm not seeing anything disproving what he stated.
Of course, it must be all lies meant to demean Ulfric's name. In the very least it's far more than anything suggesting he didn't do those things. And Nepos had no reason to lie at that point.
That he didn't even mention the deal that was struck between Ulfric and the Jarl's son (and the Empire for that matter) shows his clear bias against him. That he sits there and pretends that what Ulfric did wasn't sanctioned by the Empire the whole time shows his bias.
Hammerfell fought against a weakened dominion and won.
If stormcloaks win they will fight against a rejuvenated dominion and most likely lose because of the loses from the civil war.
I choose empire.
A weakened Dominion that had years to reinforce its position.
And no, the Dominion will not be able to win against Skyrim. Period. The terrain is far too disadvantageous and even if they managed to get some sort of foothold in Skyrim they'd be dealing with the most ferocious ground war they've ever had. Do remember what happened to them the last time a mostly Nordic army went up against them. (IE, their entire army was destroyed. The only ones that survived were lucky enough to be in the next province over) Add in the fact that ELVES are invading their homeland, and the Nord will absolutely destroy whatever forces the Dominion tries to land in Skyrim.