It's destroyable/stealable?
Yes, and quake-able. It's in a stack of it's own though, so if you have two towers and one elite mark, you're less vulnerable to quakes than if you only had towers.
To answer the OP's questions:
1. At the moment at least, you can only get one by winning a tournament.
2. They don't. The developer does.
3. If it doesn't match your mark (keep in mind that your mark is unupped), it goes in a stack of its own and can be exploded/pulverized/destroyed/quaked.
4. Marks have the same amount limits as other non-pillar cards, i.e. no more than 6 in your deck.