There is The Zombie Island of Dr. Ned, Mad Moxxi's Under Dome Riot,
The Secret Armory of General Knoxx, and they have announced that the Were working on a 4th DLC but i haven't heard any more about it.
Xbox marketplace
No.
Dawnguard , the first DLC expansion, was officially released for the Xbox 360 on June 26th, 2012.
For you to be able to play the game itself - no.
yes anything you get from xbox live will stick with your gamer tag but if you are wanting to buy it once for two xboxes that wont work
No.
its a mod
yes it's on Xbox 360, you can buy it at your local game shop or on Xbox live
dsd
Yes you can. Because the profiles of people with or without DLC packs are compatible with each other the same goes with save files in the game. in other words you'll keep your guns and stats but lose access to the maps/areas of the DLC content.
Xbox Marketplace, PS3 (idk), PC probably anywhere type Boderlands Downloadable Content (DLC for the usual xbox short)
Yes there are games similar to the DLC Quest that you can buy for the Xbox 360. You can find the similar games from game retailers such as game stop or online.