I have a BeoVision 5 which I'm told is losely based on the Avant chassis and it works for me. It is likely the STB controller is similar.
1) That item looks like it will work. But it is not necessary if you are happy to either hack your ATV or install bought software! I use mine plugged in directly using the green component video socket on the back of the ATV and the red & white analogue audio sockets. If you are good with software, there is a free download hack that changes the Video Out to composite, or you can buy aTV-Flash which does that for you and tons more! aTV-Flash is excellent and I use it for Boxee and XBMC. I wasn't too sure about doing my own thing for the software hack so decided on aTV-Flash and it is so easy to install, but if you are any good at computing it will be easy to download the free hack!
2) The STBC on my BV5 uses the settings for = Apple Remote and V-Aux. There was only one IR output on my BV5 and the IR emitter was already being used for my Freeview on DTV (SAT on the older BEO4?) so I had to buy the B&O Y splitter and a second emitter. The second emitter is a different colour on the connector to the original. You have to have one silver & one gold to control the items. Not sure if you "have to have" the B&O ones but the plug on the B&O emitters look "mono" as oppposed to the "stereo" style on all the ones I could find on the net so i didn't want to chance it.
Take a look here for the aTV-Flash details http://support.firecore.com/entries/259319-using-composite-video-output
& here for the various free hacks http://wiki.awkwardtv.org/wiki/Composite
These instructions will work for any TV with a scart connector too!
Good luck
Stoobie