Yes. Have you searched this forum, or Google, on these topics? They are all very prevalent with a LOT of content regarding these things.
Yes you can downgrade if you have BLOBS. Use Snowbreeze. That said, there is now a jailbreak for 5.0 in ATV2 and, as of yesterday, there is an EDEN release for it.
Have you attempted to load XBMC without using aTV FLASH?