1. Germany didn't have the ability to create fire at their fingertips...
2. If they successfully took Skyrim, that would be leaving the Empire. I'm not saying they would, but it would be a nice opportunity to weaken your enemies forces no?
3. A Legion is waiting at the Pale Pass.....
1. Not all the Aldmeri are skilled mages. Regardless, Skyrim still has home-field advantage. The common Aldmeri soldier isn't a heartless, emotionless killing machine. They're far from Alinor, freezing, and up against fur-clad, battle-axe wielding, war-paint adorned screaming hulks, who are right at home.
2. Skyrim and Cyrodiil aren't enemies. Ulfric refuses to attack Solitude during the Emporer's visit. The rebellion is to allow Nords to live the way they wish, if you're familiar with Redguard, their rebellion there is to ensure they are treated better by the empire, not to overthrow the them or completely sever ties. And why would Cyrodiil sandwich themselves between the Dominion and Skyrim? I must be missing something here.
3. How do you know? If this assuming Bruma's fallen, why are they standing around at Pale Pass, I'd be um, I don't know, fighting the dominion? Also Pale Pass is a mess at the moment, avalanches and such.