1) you know this how, considering men are present there.
2) Nord success isn't dependent on invading Summerset. Its only dependent upon driving out the AD. Miring down the AD on multiple, unbeatable fronts (Hammerfell and Skyrim) is all the empire really needs to recoup and counterattack. This is why cutting the chains on the Nords is the best thing that the Imperials could be doing.
Again, this is exactly how the US bankrupted the Soviets during the cold war. Proxy war via supporting the Mujahideen.
The Thalmors goal is too destroy the world and ascend to a higher plane of exsistence. In order to prevent that Alinor must be taken eventually.
As far as I know, hasn't it always been a big deal Summerset Isle is a xenophobic Altmer only state? I may be mistaken. There may be men there, but not actual Empire territory to foot an invasion from.
In the long term the liberation of Skyrim is a priority, but in the short term I think that it is in every mans (and some Mers) best interests that the best Nordic armies are at the front trying to take Valenwood or break off the offensive on Hammerfell. It is easier to drive an occupying army back with a united force rather than various states concentrating on defence.
Also, Alinor is a country. It is more than capable of supporting itself (and more importantly it's navy). It isn't communist, therefore there isn't the hit to productivity that the Soviets suffered from. Not to mention that the economy in Tamriel is different.