skyrim lead by the stormcloaks could have the strength to protect it's people.
For now, yes. But the Thalmor don't care about short term. What happens if Cyrodiil falls? Then Hammerfell will be wide open and 50 years down the line then it will be Hammerfell's turn to get overrun, sacked, and burned to the ground. Then expect maybe 100 years of consolidation, while the few surviving humans in the now greatly expanded Thalmor territory are systematically broken down and made slaves, and only then will it be time to deal with Skyrim. Then the scheming and spy-games start, key people are assassinated, army orders are falsified, key farms end up with poison in the feed for the livestock, a plague will spread from a city well somewhere, and all of a sudden there's going to be Thalmor buggers everywhere.
The Nords will fight bravely and couragously, but city after city will get sacked, farm after farm will get raided, livestock will be slaughtered, and eventually the Nord resistance will be starved to death. Meanwhile the Thalmor can simply move in their supplies from Cyrodiil. The Nords might be able to repel the invasion, in which case there will be constant raids across the borderlands, massive piracy by sea, and doing anything to stop this means invading Cyrodiil and Hammerfell and kicking out the now heavily dug in Thalmor.
In the short term, Skyrim will be safe under the Stormcloaks but then it would anyway. If you look at a map then there's no suitable invasion route for the Thalmor to attack Skyrim. No matter what, Hammerfell or Cyrodiil has to fall first and that won't happen while the Empire is still intact.