No, 3DS games only work in a 3DS or 3DS XL.
Yes, they will since the 3DS XL is a 3DS.
No, 3DS games only work on the 3DS and 3DS XL as well as the 2DS
In comparison to the 3DS, the 3DS XL is much larger with a bigger screen. The 3DS games work just the same in both consoles.
Yes, the 3DS XL can play Nintendo 3DS games
No. 3DS games are not compatible with the DSi XL.
Yes, the 3DS are backwards compatible.
Yes, the Nintendo 3DS (as well as the 3DS XL and 2dS) has its own 3DS games that will not work for the Nintendo DS, Nintendo DS Lite, Nintendo DSi, and Nintendo DSi XL.
There are no '3DS XL' specific games. There are only '3DS games'. The XL is simply a larger model of the 3DS, not a new device.
You'd have to use system transfer to migrate your ID from the 3DS to the 3DS XL before you can access your downloaded games on it.