1. AMR -> other formats. Use QuickTime. Guide on link below. Haven't tested it myself.
http://www.gromkov.com/faq/faq2004-0087.html
2. Agree. Wouldn't expect SonyEricsson to be the first to release one though, since I suspect they want to push their own stuff first. One of the features of W800 over K750i is that W800 comes with better headphones.
If you haven't noted, the handsfree car-kit will connect the K750i to the speaker system of your car stereo, complete with phone muting. Don't know any more details about it though.