Lol no. supporting Ulfric =/= supporting Thalmor. Like I said previously, Ulfric has already shown a willingness to deal with the Imperials for Skyrim's best interests;
And when exactly did this happen? The peace conference
I remember was Ulfric deciding to use a world-threatening crisis as a perfect opportunity to screw more concessions out of the Empire. The dude threatened to take his ball and go home and let Alduin the World-Eater rampage over everything because he
knew that Tullius would have to back down rather than actually allow that to happen.
IOW, Ulfric took advantage of the fact that his opponent would be concerned for the welfare of Skyrim/Tamriel as a whole than Ulfric was. Which says a lot about Ulfric, and none of it good.
there's nothing to suggest he'd try and take them alone after winning his independence. More likely it'd be a case of 'okay we got out country back, now.. who wants to kill some Thalmor?'
Yes, because their odds would be
so good with Skyrim all by itself. Especially a Skyrim weakened from a recent civil war.
Also, did you read 'The History of the Great War'? The main thing that let the Empire break the siege of the Imperial City was the arrival of the Nord expeditionary force. With Hammerfell gone, Skyrim is the last major military province the Empire has
left. What does the next act of the Great War look like without it?
And before you go 'But a separate Skyrim could still ally with the Empire!'... no, no he couldn't. Listen to the Stormcloak agitprop. Ulfric is selling this to his people on a platform of '[censored] the Empire, the Empire is the Big Bad'; he's burning all his bridges there. Hell, the Stormcloak party line is that the Empire and the Thalmor are [censored] buddies.
There's also that the Empire would trust Ulfric since whenever? He's not exactly making a good impression with them, now is he? Even if Ulfric wanted to buddy up with the Empire next decade (and that's a
huge freaking if, right there), its still anything but certain that the Emperor would ever go for it. Because let's face it, Ulfric has -- and this is on the record -- sold out and backstabbed every single side he's ever worked for, except his own. Man betrayed the Empire in the Great War, then he turned on his Thalmor handlers afterwards, then he murdered the High King, then he goes on to rampage against neutral jarls. Would
you trust this dude in a political alliance ever? How could you? Every friend he's ever had got an axe buried between their shoulder blades.
Sure, the Thalmor ambassador's report says that a quick victory to the war is sub-optimal; they want both sides to weaken each other to the maximum extent before it finally ends. But in the long term, a Stormcloak victory is a human defeat, because what fought the Aldmeri Dominion to a tie the last time was a united empire -- a divided one can only improve their chances.