If I delete my passwords every session in Firefox, can I set xmarks to restore my passwords automatically when my PIN is provided?

This isn't currently easily done with Xmarks password synchronization; in fact, if you set Firefox to forget passwords, and Xmarks synchronizes those changes, you'll effectively lose your password data everywhere.

There is a way to get this general effect via some complicated manual steps, but I wouldn't recommend it due to its complexity and the strong likelihood that you'll end up deleting your passwords on the server (and subsequently on all other browsers you're synchronizing with).

The way I don't recommend involves disabling password sync, then occasionally enabling it to download (NOT synchronize) your password data, then immediately disable password sync after that. Again, not a good way of achieving what you're looking for, unfortunately.