My thoughts... (sorry if not too much help)
1. I think the issues are USB related, particularly on resume from standby. Some drivers take longer to restart and StreetDeck tries to open COM ports right away - I think they get deadlocked somehow.
XM used to be my number one complaint until I resorted to a standard serial cable (bought from some nice guy on this board who made one for himself plus 10 more). Now no issues with XM.
2. That is weird. Streetdeck by default uses the spacebar to initiate voice recognition - you aren't sending it a space character using girder by any chance?
3. I did not know you could rewind an XM song - cool! The gestures work as expected as do my radio controls so I never noticed this one.
4. I have the SPDIF hack (again from someone on this board) on the XM Direct unit feeding to a SB Extigy. The sound quality is amazing. That said, the Extigy has some "issues" but in general is not a problem for me anymore. The good part about it is that even if the PC is off it can still function, routing signals from the stock HU into the AMP (so you can listen to the CD, AM/FM, etc).