Again the Redguards aren't stupid, they are a great and smart people who have had many great tacticians to grace their race and warfare and violence is like second nature to them, they are a honorable people and skilled warriors, they know that when it comes to destroy a great enemy pride must be put aside.
I agree with Treng here, you are ignoring a lot of Hammerfall her history. The only reason they are part of the Empire is because a civil war weakened one of the two mayor sides so much that they begged the Empire for help. The chances of people like that going in and saying, we want to join you again especially after the recent betrayel are incredibly small. From what we know of Hammerfall today they would never ever do it. They might be willing to form an alliance against the Thalmor though, if they can get over their pride for being given away.
That's the best you can hope for, well that or a war in Hammerfall that will bring the province back into the Empire, but that will just cost a lot of lives and troops and would give the Thalmor a perfect reason to attack as it can be seen as a breach of the WGC.
No one from Bethesda said that independent provinces are the only way to defeat the Thalmor, I'm speaking from the side of tactics and it still clear to see that an empire is a better way to preserve some amount of order then independent nations.
Why? The Empire has to worry about a lot of area's. The Imperial province, High Rock, possibly Hammerfall, Morrowind, Skyrim. An alliance of independent provinces would be far better equipped to deal with the Thalmor in my opinion. Primarily because a province like Morrowind could be ignored, and you won't lose troops there, you can also ignore local politics as many Nords would be more willing to fight with the Empire then under the Empire and the same would go for Redguards.