To answer number 1: yes, you can get by with just the spdif, the 360 passes out what it gets, so if the source is 5.1 that's what it passes back. WMA files that are encoded with surround audio seem to get converted to 5.1 dolby. At least that's what my receiver recognizes it as.
The only annoying thing I've found with the 360 is the limited sensitivity of the IR receiver for the remote. You pretty much have to be pointing your remote at the 360 for it to get the command.