No he is forced to join, because Ulfric won't make peace. He says he takes no sides. Have you spoken to him about the CW lately?
He really gets pissed off even having to choose the imperials. As far as I can tell, he only has a beef with Ulfric. Which doesn't necessarily mean he wants to side with imperials.
The Jarl of Whiterun obviously accept the Empire at the condition they don't militarize the town. That doesn't make him neutral. The Empire just has to insist a bit to be able to place their troops in the city, while Stormcloaks have to take Whiterun by the force. If that's neutral for you, you'll have problem to understand geopolitics.
And again I'm talking about the power of Thalmor. Where is it? They are unable to make Whiterun respect the white gold concordat, their attempt to take control of the Winterhold College is pathetic : just the egomaniac power trip of one of them when there's the occasion.
So seriously, where are they? Where are the Thalmor?
They have no way to back up their claims of supremacy, and that's why they're so pathetic.