I know its already been asked and answerd, but im too lazy to go look so;
Where in the name of Talos is Arvak's skull?
It's directly East (Possibly NE) from the big building where you first meet Serana's mom. You'll see a gazebo type structure with Mistman surrounding a pedestal. The skull is sitting on the pedestal. Good luck.
Can you do the Dawngard side quests (Like the Dwemer crossbow etc) after doing the main quest for them?
You can do them during the main quest, as soon as you want to, in fact.