For the most part, yes. Occasionally you will see some DLC segments released on disc, such as Fallout 3 GOTY edition, which comes with all the added on story segments, or Rock Band also releases discs with a certain type of songs on it originally offered as DLC.
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
yes it's on Xbox 360, you can buy it at your local game shop or on Xbox live
Xbox marketplace
No.
Dawnguard , the first DLC expansion, was officially released for the Xbox 360 on June 26th, 2012.
you can't only ps3 and xbox 360 can DLC
yes,but maby he is only for xbox 360.
No.
The DLC is saved to your hard drive, but only the account you downloaded the DLC with can access it from the hard drive.
yes, you can play DLC that was downloaded on a Xbox LIVE profile and play it on a non Xbox LIVE profile
dsd
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.