There is no reason to bate people like that Kael, as much fun as it is to say
As for the Altmer navy vs a Skyrim one. I cant find the book detailing it online but I remember a book in Morrowind that talked about the Nords and Redguards being the masters of sea warfare. I might load up Morrowind during winter break and look for this book. It is stated in the Pocket Guide to the Empire for the Summerset Isle that they built a great navy to combat the Sea Elves. But after the Dominion was beaten and the Isle taken into the Empire. The Sea Elves invaded and nearly captured the entire island, only with aid from the Empire and the Psijic saved the Isle. So I not 100% sure the Dominion has this unstoppable navy. The Nords on the other hand have always been known as competent sea men and you would be hard pressed not to find one in every port and coast of Tamriel.
The occupation of Hammerfell was only the part given to the isle by the Empire, which was the Southern Coast. I haven't seen much information on this war, but it seems it only really took place on the coast. It also seems the Redguard fought the Dominion with guerrilla tactics that was so costly to both sides that a peace treaty had to be signed. Which expelled the Dominion from Hammerfell. If the Dominion had the ability and resources to hold the Southern coast I see so no reason why they shouldn't have. I see that this shows the Dominion is far weaker then they let on.
I'm a little at a loss of how many Legions the Empire actually has. We know 2 unnamed legions and the 8th were destroyed during the war. Other wise we know the Empire had 3 armies in the battle of the Red Ring. One led by the Emperor, composed mainly of Imperial Legions. One under General Jonna made up of Nordic legions and then finally General Decianus legions from Hammerfell. The part that confuses me are quotes like this
Unwilling to abandon Hammerfell completely, he allowed a great number of "invalids" to be discharged from the Legions before they marched eastThe second army, largely of Nord legions under General Jonna, took up position near CheydinhalThis implies more than one legion per General's Army during the Battle. Also What happened to the Legion in Morrowind? And is there ever any mention of Legions that come out of High Rock? Also in a 30 year span I think the legions would be back to its original numbers. And in the best case scenario the 3 legions destroyed would probably be in the process of being rebuilt, maybe even to half the size of a normal legion. The recruitment in Skyrim only lowered when the Civil war started. I doubt the Altmer could make the same claim. Unless as you suggested the Dominion actually makes use of the Wood Elf and Khajiit under their rule. I'm kind of skeptical on the Dominion using either as anything more than servants. But I wouldn't be surprised if we see some lore or actual Bosmer soldiers in the future of Skyrim and be proven wrong
As for Skyrim being able to stand up to the Dominion. I found this little fluff piece from the Pocket Guide to the Empire, 3rd Edition/Hammerfell to be interesting.
Eastern Hammerfell, less concerned with Sentinel, has continued its efforts to take back the lands conquered by Skyrim in the War of Bend'r-Mahk. There is no question that Redguards, while currently lacking the cohesiveness as a society to form effective armies, are excellent warriors and unmatched at guerrilla warfare. Still, little ground has been regained, for the Nords too are renowned warriors.
This shows the Redguard even lacking in unity drove the Thalmor out of Hammerfell. Yet they cant reclaim territory from the Nords. An occupation that happened in 3E 396 and the Redguards haven't retaken any of that land in the last 200 years? Where as the Redguards fought the Dominion to a stand still and eventual expulsion from Hammerfell in 5 years.
The desert of Alik'r may take up the majority of Hammerfell. But all of the towns and cities outside of Skaven and Dragonstar are located on the fertile grass lands of the coast line. Where the majority of the war in Hammerfell was fought. This enviroment would be far less treacherous to invade than one like Skyrim's. Then again We don't know what the flora and fauna are like, they might have predators in Hammerfell just as bad as Skyrim's.
Compared to the rest of the Provinces I would say Skyrim is either number one or two on the dangerous to live scale.
If Skyrim gained independence, I feel the Nords would act quickly and start to harass the Dominion by raiding the coasts and attacking trade ships. If they really got cocky they might push in land, raid and then leave and then come back at a later date and different town. I'm also fairly certain the Redguard would join the Nords in these raids if not out right form an Skyrim/Hammerfell Alliance and actually invade the Isle full force. I would like to think the Dominion would crumble under the might of two of the most renowned warrior races on Tamriel, but these things are never certain. And no I'm not saying Orcs aren't renown warriors as well ya nit picky Orc players

Orsinium and many Orc settlements really don't want anything to do with anyone.
And if the Dominion were to invade the Empire. I think the Nords and Redguard would join in. Relations would be fairly terrible between the Nord/Redguards and the Empire but the enemy of my enemy is my temporary friend

And even if the Nords and Redguards don't jump into the fray they could still do what I suggested earlier. By raiding the towns and coast lines of the Dominion would cripple the main war effort and the need to divide forces to defend the homeland and its people. Could you imagine the rage of the Thalmor that dirty humans would dare tread on their land?
Also I thought the goal of the Thalmor was not genocide but subjugation and dominance of the Elven races?