Marriage doesn't need an EFB, no. Because the husband or wife wouldn't be *your* character, they'd be an NPC, so it would be perfectly acceptable to say the spouse was a pureblood if that's what you wanted. However, if your character had a child with that spouse, and someone else later created the child and played them, this other person would them need to create an EFB for the spouse's line.
Necromancy is on the Durmstrang curriculum, so it's absolutely fine for now to say your character learnt it at school and that it was a subject they enjoyed and got a good grade in it. Then later you can apply for them being an actual necromancer.
I'm pretty sure Azkaban is *the* wizarding prison, at least on Europe. When Karkaroff was on trial after the first wizarding war he was on trial to potentially be sent there, so certainly there's scope for folks from the Durmstrang area to be sent there. At that time, they seem to have all been kept in solitary rooms from the descriptions in the books, though since Voldemort's downfall there aren't dementors there any more things are rather different (at least here on MH) so it depends within a little when you want him to have been imprisoned.
Hope that helps a bit!
[carys]~ Carys