Update on my parasitic battery drain:
After two bimmer mechanics couldn't pinpoint the problem (the second was 90% sure it's the instrument cluster), I've had the X5 sitting in my garage for about three weeks with the 5-amp Fuse 75 pulled (MID, Nav, On-board Monitor, Sound System) and it's started/run just fine after one-, two-, three-, and four-day "sleep" intervals -- so I think I'm onto something. I went back to the MID because of the intermittent "ramp-up" sound I'd hear when in the garage and the car was supposedly asleep. It's like the MID starts its initialize process randomly, without the door(s) or anything being opened.

So, I'm assuming I need to swap-out the radio/sound system unit (if I want my music back!). Anyone got a good source for used/pre-owned bimmer sound systems? Thanks.