So I apparently can't link to it directly, but you see in Elder-Geek's youtube video "The Elder Scrolls Online Realm vs. Realm Preview" that you can move Elder Scrolls from keep to keep in order to better defend them, but they have to be carried by a player who can be intercepted.

What's to stop other alliances from using alt characters in your alliance to walk into your scroll fortress, take the scroll, and walk it to their alliance base and letting themselves be killed?

you cant have a char in the same campaign as a different char you own.

Well assuming it's the same as in other similar games. Then they'd be unable to pick it up from where it was placed.