You want to persuade me to believe in what you think? If such is the case then pardon me for throwing a minor essay after you, but you did say you wanted to convince me. And convincing me is somewhat word-intensive, I'm afraid. Sorry.

The Summerset Isle is by no means a large province, infact it is one of the smaller ones. And the Aldmeri Dominion does not have much military support from Valenwood or Elsweyr. Nor do the Thalmor have an army THAT large, their victory throughout most of the Great War is credited to tactics and planning.
This much I agree with. What you should not forget, however, is that Summerset Isle hasn't been consistently plot nuked for the past three TES games and in-between. Summerset Isle is furthermore largely a fortress that is ridiculously hard to invade. Finally, as you say, tactics and planning. But you're forgetting one thing: Vastly superior with respect to espionage and covert manipulation. They out-spied the Blades, despite all the resources the Blades had. The WGC simply allowed them to open an unlimited hunting season for those they knew to be Blades operatives, but the Blades were already hurting before that time.
Afterward, Titus Mede II presented the Dominion with the White-Gold Concordat Treaty. Mede II had over estimated the Dominion, and ironically, the Dominion had done the same. If they were powerful enough, they would've proceeded to destroy the Empire, regardless of a treaty. Even though the treaty heavily favors the Dominion, it ultimately detains them from reaching their ultimate goal.
I disagree with your conclusion here. The Thalmor had just lost their entire invasion army. It is obvious that they couldn't rebuild a new army and invade again right away. Even so, there's this approach called border raiding. They could've repeatedly sent small forces into Cyrodiil to do quick raids and run back out again. The border between Cyrodiil and the Thalmor-"friendly" lands of Elsweyr and Valenwood is too large that the Legion can lock down all of it and there'd also be the possibility of naval insertions. This would be something the Thalmor could do pretty much unchallenged, and the Empire would not be able to stop it, since doing so would mean mounting an offensive on Valenwood and Elsweyr. Considering the already serious level of war-fatigue at the time, such a prospect would be terrible.
The Thalmor, meanwhile, have terms that more or less allow them to do what they initially wanted to do. They're not bound by the peace treaty any more than the Empire is, and the moment either side see a decisive advantage in renouncing the treaty and going to war, they'll do so and not give a damn about the treaty. Effectively, the treaty is largely worthless. What really matters is that the Thalmor and the Empire have agreed to put aside total war for now. The war is still there, both sides know as much, but it's a cold war currently. The Thalmor can't allow the Empire to prosper and the Empire can't let the Thalmor win, since "Thalmor win" effectively means "Empire loses".
Why isn't the Thalmor attacking already? Because they clashed with the Legion and realised that it was too big a mouthful. When you have all the time in the world, there's no reason not to play things safe and manipulate conditions to favor you as much as possible, before you make your move. That's what the Thalmor are doing. If the combined Empire is too strong then the logical thing to do is to divide the Empire into smaller pieces. During the Great War, the Empire was Cyrodiil, Skyrim, High Rock, and Hammerfell. Now Hammerfell is out and Skyrimg is trying to get out as well. What was a big empire back then is a much smaller empire now.
The Redguards were outraged at the Empire for paying the treaty with their hard-won lands and waged their own war against the Dominion. The Empire was forced to renounce Hammerfell as a province, and a war between the Dominion and Hammerfell would be waged on for five years. Now, keep in mind that during this time the Thalmor had no other pertaining Military threats, Hammerfell was their only target for their offensive military. The rest of the Thalmor's mer-power was directed toward defending borders and rooting out Talos heretics throughout the Empire. The war was fought to a stalemate and eventually the Dominion had to withdraw their forces from Hammerfell. What does that say?
Again I think your conclusion is a bit off. The Dominion has just gotten an ass whopping at Red Ring but prior to that, they prison-[censored] the Imperial City, capital of Cyrodiil and the Empire. That's not a situation where you trust a peace treaty all that much, which means it's also not a situation where you throw everything you have into the invasion of some "inferior province". The Thalmor assembled an army of what was left after the invasion force got wiped out, and I refuse to believe they emptied out their lands to do this, which in turn means the assembled army was mostly reserves. Even so, it fought against possibly the most fierce swordsmen of Tamriel for five years and still didn't get destroyed. And this was a reserve army.
Why don't I think they threw everything they had after Hammerfell? Because the Legion was still alive and kicking at the time and if they had gambled and gone all in then the Legion could've joined the fun by sneaking along the coast and effectively encircled all the remaining Thalmor forces. Then it would've been Red Ring 2, but this time it would've been devastating to the Thalmor. After this, the Empire and Hammerfell would easily make friends again and an invasion of Elsweyr and Valenwood would be next. This can't currently be done because both provinces are somewhat heavily garrisoned, but remember that the premise was Thalmor sending everything they had into Hammerfell. Surely that would thin out the garrisons. Since the natives of both provinces aren't hugely fond of the Thalmor, it's very possible that the provinces would fall. That would also remove the Thalmor foothold on the mainland.
What makes people think that Skyrim, home to the other group of strongest, fiercest warriors Tamriel has to offer, would be crushed by the Dominion without the Empire?
Because the conditions would be vastly different. It seems unlikely to me that an invasion of an independent Skyrim would take place before Cyrodiil and Hammerfell have been completely broken. It may be 200 years into the future, but operating with that time frame isn't a problem for the Thalmor. And once the Thalmor control pretty much everything south of the Skyrim border and west of Black Marsh, I really don't see how an isolated Skyrim can put up much of a fight.
-Men breed much faster than Elves.
Several generations have passed sicne the Great War (30 years worth), and even though Elves live much longer than humans, they populate a lot slower.
The great war ended in 4E175. Current time is 4E201. That's a mere 25 years after the Thalmor captured most of Cyrodiil and Imperial forces recaptured it. 25 years isn't a long time, not even for humans, not even if they breed like rabbits.
it depends on what you think a 'big deal' is. In my opinion, Thalmor Justiciars going to Talos shrines and slaughtering innocent people (sometimes not even Talos worshippers, just those who happened to be near the area), Nord families ratting eachother out, Thalmor imprisoning, torturing and evnetually killing 'alleged' Talos worshippers without any solid proof and the Empire not doing a damn thing is a pretty big deal.
Whiterun is Imperial and I've yet to see any Thalmor drag that Talos priest away. The Justiciars are not really hunting for worshippers anyway, they're simply doing intelligence-related wet work. Of course, due to your friend Ulfric, they're largely forced to make a show of it, since he went out of his way to demonstrate how the Empire wasn't at all enforcing the ban. If Ulfric had not made all that noise for no gain whatsoever, the Thalmor wouldn't have had a diplomatic incident they could use for leverage to demand concessions and things would be much better for everyone.
Effectively, Ulfric screwed over people living in all parts of the Empire for his own gain. And please don't give me crap about how he has the vision of a free, strong Skyrim. Well, he may want that but he wants it under his leadership. And that's not going to be good for SKyrim, because he's not actually all that bright.