1. True enough, but don't forget that the Thalmor are also rooting for Ulfric's war. The moment they see him winning they'll probably pull back and set the stage for their attack.
2. It's perfectly valid, but Bethesda's writers just don't use it when they can. Unless there's a counter magic to that of course, never heard of it though. Marching to Skyrim isn't impossible though, and no, they wouldn't get smashed by Cyrodiil. Cyrodiil have no business in Skyrim anymore and can't afford to break the treaty again.
3. Sure. But all before that it was the Aldmeri Dominion kicking ass and chewing gum. Cyrodiil won back Cyrodiil, not Tamriel. The Aldmeri Dominion currently occupies most of the south whilst the other Provinces are still struggling aside from Black Marsh. And forget Morrowind, that place is screeewwweed. Cyrodiil won a battle, a victory needed by the edge of defeat.
That does not mean they are fully capable of fighting back immediately and alone.
The more Provinces back out the worse it'll be in the long run. Hammerfell did well on its own because they're the realist niggas on Nirn and can kick ass all the time (apparently) but the Aldmeri Dominion continues.
In fact, I'd put my money more on Hammerfell than Skyrim. Skyrim is really screwed either way, just less screwed under Imperial government. That long ass post already explained why a Stormcloak Skyrim would fall flat on its ass.
P.S C'mon! When you have the bloody THALMOR rooting for the Stormcloaks it's BETHESDA telling you 'yo, this ain't good.' Yeah, under Dovahkiin they stand a preeety good chance at surviving. But everything I've said up to now is a Stormcloak land with no FUS ROH DAH in sight.
He's gone to Elsywere in the promise to find beaches and furies with loots of skooma.