After completing the Stormcloak side of the Civil War, Ulfric and others were mentioning 'taking the fight to the damn elves' or some such nonsense.

If the small Stormcloak army could barely get out of a stalemate with a weakened Empire - a fraction of its legions at that - how do they expect to take on the combined Aldmeri Dominion?

A direct path is not accessible for the Nords in Skyrim either. They'd have to go through Cyrodiil before anything. A sea route would be possible, but they'd have to go around Highrock, Hammerfell, and face the obviously-more-powerful Thalmor navy.

They also seem to mention 'Skyrim will lead mankind through those dark times'. Is that implying they look to ally with the Bretons and Redguards, and even possibly the Imperials? Do you think the Empire and Skyrim would ever ally in the near-future? They share a common enemy, and the only bad blood caused between them is from that damn White-Gold Concordat.

I don't understand why the Empire just didn't let Skyrim secede from the Empire, then ally with them right after, and together face the Dominion. It'd save resources, men, time, and would not cause bad blood between them.

Because having a provice is tax income and access to natural ressources such as ore, wood and men.
You dont surrender a provice to a rebellion until the costs outweight the benefits.

So you're saying the Empire should waste money, resources, and men in Skyrim so they can keep getting more money, resources, and men from Skyrim?? I just think it would be better to let them secede, initiate trade with them, then ally in the next Great War. The soldiers in Skyrim could very well be garrisoned along the Cyrodiil-Valenwood/Elsweyr border.

Human history simply proves you wrong.
Colonial Empires fought against rebellions and popular uprising even though the colonies had long stopped giving any real benefits to the Colonial power (just google decolonization in the 20th century).
You simply do not conquer territory and then retreat as soon as some idiotic guy in fur shows up and claims himself king.

this is true

i think the SC's are speaking metaphorically that they will oppose them and if not... well it's been fun Tamriel but im moving to Esroniet, there there aren't any elvan overlords there.