Actaully Ulfric and Skyrim wouldnt have to go thru hammerfell because Nords are remarkable naval fighters as well, pretty much renown as the best, AND Ulfric as alrdy stated that they will go after the Thalmorf they have to carve thru the empire to get them...that sounds like pretty much even the empire will have an ally when i comes to the thalmor, at least just for the fighting. Yes the Empire should figth til its dead in the eyes of Ulfric BECAUSE that is how a Nord fights, u fight well and u die well..thats ti in battle, dosent really mind who wins/loses as long as u gave them hell and went out swinging. Also as far the Dunmer, if u actually read its the dunmer's fault for the separation of them. When they were invited to come over and stay in skyrim, they came over and was very disappointed thinking that they were gonna receive a handout and that they would have a place where people were tolerate/respectful of Morrwowinds values/ways, but got really upset whenthe citizens didnt really care about their "old" homeland values and stuff, just wanted the Dunmer to respect the ways of Skyrim, the dunmer were really put offby the lack of morrowind tradition in skyrim "big surprise" so they started forming thier own pubs and shops fashioned after thier old homeland and followed the culture of thier old homeland and pretty much disrecarded the traditions and ways of the very land they live. The dunmer themselves created that rift.
Also pretty much when the war is over, the cities are for the most part already getting back to work, and u also cant forget that this is the Nord race, where fistfights and battles are a FAVORED pasttime for the general population. Nowhere does Ulfric state that the empire should completely die, only that it shoulda went down fighting with all its might during the great war, but then again thats EVERY nords point of view. TBH i cannot find any evidence that a skyrim/cryodill/high rock empire would be able to do better what the old empire woulda been like in the old war. Now u get the individual countrys as shakey alliences that are united in a common enemy that they all hate....yes i could see it happening. Also the Empir eis waiting for itself to gather up more strength to fight, well cryodill can do that while the Skyrim nords are doign guerilla warfare on the Thalmor's HOME ISLAND, remind the high elves that nords are considered the BEST naval warfare race.
The nords are not regarded as the superlative naval fighters in modern Tamriel. The Thalmor managed to gain a foothold in Summerset Isle due to their tactics in dealing with pirate reavers from Hammerfell and Valenwood. Now they control Valenwood and Hammerfell is independent. In addition, swinging around High Rock to deal with the Thalmor is a stupid, stupid plan. The Thalmor would have advanced warning, it would require a great sacrifice of domestic defenses and any ships they took would need to carry more food and supplies than any vessel departing from Summerset, Valenwood, or even Elsweyr.
Regardless of who wins, the Stormcloak rebellion costs many, many nordic men and women their lives. The survivors might indeed return to their hometowns but there are those who will be brought back in pieces, others who will never return in any form. This is the price of war. There is no way the people of Skyrim would ally with the people who culled their sons and daughters even if they did have any respect for non-nordic values... which many don't. Likewise, Cryodiil, High Rock and the nords who served in the Imperial army would feel betrayed and would also feel the same sense of human loss. To say nothing of the issue over who would lead a united human front. Only nords would take orders from Ulfric and there are fewer nords now than there were before the rebellion.
When it comes to the dunmer, Ulfric is asking them to forsake their gods, their culture, their loyalties and everything that makes them dunmer in order to gain respect. Ulfric fully believes that Skyrim is for the nords. He is intolerant of opposing cultures, apathetic towards those which aren't antagonistic but the dunmer are denizens of Skyrim and he refuses to deal with them as dunmer. Outside of Windhelm, many of the dunmer players can meet are settling well despite not having the same beliefs as those around them. There is a problem in Windhelm, a problem Ulfric continually ignores and that his allies are keen to amplify.
What's 'canon' is a different issue altogether. Since we get a choice of who can win the Stormcloak rebellion, it's safe to say that no one wins in the end. Something has to come along and crush or aid both armies otherwise half the playerbase would have their playthroughs rendered invalid. As a result, I personally believe that the Empire falls apart and think it's likely that the Thalmor pick up the pieces.