The real question is what has General Tullius and Jarl Elisif the Fair done about the Thalmor Embassy and the Fort? Since it's right on the foot step of Solitude. What do you expect Ulfric to do while the Thalmor Embassy and the Fort is resting comfortably on imperial territory? If Ulfric decided to invade the fort and the Embassy he would not only have to deal with the Thalmor, but also the army of the imperial legion. 2V1 is not a fair fight, and Ulfric knows that would be suicide. If the Empire was smart enough they would've join forces with Ulfric and his army to fight against the Thalmor. Instead the Empire sent General Tullius and his soldiers to deal with Ulfric and his rebel soldiers. NOT A SMART THING TO DO.
I believe what Ji Soo was referring to was the lack of action taken by the Stormcloaks against the Thalmor
after they have won the war. This means that Solitude is in Stormcloak hands, so I assume they will meet no resistance from Imperial forces. I am assuming you merely misread his post.
Also, the Empire and Stormcloaks would never join forces due to the fact that their goals run counter to eachother. The Empire wants to destroy the Thalmor to reassert their power in Tamriel, whereas Ulfric wants to destroy the Thalmor so that Skyrim can become completely independent from outside forces and to exact revenge. Obviously, the Empire does not want a party within Skyrim to rebel since they need the resources to fight the Thalmor, so sending someone to stop said rebels, in this case Tullius, is a perfectly expected and reasonable reaction.
No Ulfric is not afraid to go after the Thalmor. The empire is since they got their asses kicked by them so their tail is between their legs with the Mers.
Read this, then read it again and again until you know every single fact.
http://www.imperial-library.info/content/great-war
Here is one particular detail that I think you should focus on;
"In the end,
the main Aldmeri army in Cyrodiil was completely destroyed. The Emperor's decision to withdraw from the Imperial City in 4E 174 was bloodily vindicated."
Now you might be asking "Then why did they sign the WGC?" right about now, so let me explain. The White Gold Concordat is a
temporary peace. It does not in any way guarantee total vassalage on the Empire's part, nor did it originally grant permission for the Thalmor to roam freely to arrest Talos worshippers. What it did guarantee, atleast what we do know from the small amount of information it had, was that it ceded Hammerfell to the Dominion, outlawed Talos worship in the Empire, and an end to the war. When Hammerfell resisted, Titus was forced to renounce is a province in order to preserve the recent peace. Until the Markarth Incident, the Empire enforced the ban on Talos worship. Of course, the ban was entirely ignored and everyone kept their shrine to Talos, as Alvar would tell you. I do not think the Thalmor can wipe out Talos by kidnapping worshippers one at a time, so it is paramount that the Thalmor do not defeat the Empire or any of the provinces otherwise they can eliminate Talos worshippers at a far faster rate. The purpose of the WGC was to allow the Empire to build up for the next Great War, so by maintaining peace, albeit a very cold peace, the Empire can increase the chances of it and humanity achieving victory.