Not at this time.
No, original Xbox controllers do not work with the Xbox 360, so any games you download from Xbox Live Marketplace will use the Xbox 360 controller.
Yes, all the content and add-ons can be found on the 'Marketplace'.
Not on the Live marketplace, but I'm pretty sure that it will be on games on demand.
yes
download them from the marketplace on xbox live
If you buy something on XBox Live, games, add-ons, (everything except Avatar items) will be shared with all the profiles on that xbox, you can allways re-download it for free on any other xbox or location you are in, but will need to sing in in your windows live ID
No, you can't do any Xbox live-related actions while playing original Xbox games, downloaded or not.
none
yes you can download cool stuff on it
XBL stands for Xbox Live, and is Microsoft's online service that allows xbox users to play games online and download content from the marketplace.
You get on Xbox Live then press start in Halo 3. You should see Marketplace. Look in Marketplace and pick which maps you want to download. Then download them.
If it's on the Xbox Live Marketplace then no.