1- As Jabir said, Durenmar sits well inside the frankish empire, while Saxony has never been touched by "civilization"... although today both of them are parts of Germany, it was much different back then.
2- Being an early center of Hermetic Magic doesn't mean that it was easy.
3- As I said, I don't strictly follow the canon, as I have a much more troublesome path for the Order to walk.
Yes, more vitkir than in a regular campaign, but not a very lot of them. I'll have many different kinds of magic users, and vitkir are just a part of it.
Again as jabir said, we're not talking about Vikings, we are talking about Saxons (who share mostly the same culture, but are not vikings). Saxons are fighting Charlemagne as the Franks try to conquer their land, and vitkir are fighting alongside their people. So they have their own big war to fight, and don't need another enemy. The player characters may decide to side with the pagans or with the christians or remain neutral... this decision will determine if the vitkir will be friends, enemies or neither.
We'll see how the situation develops, but I am definitely changing some of the classic setting. Here's the introduction to my saga, if you're interested: Domus Parva 790 A.D. - alternate setting