The really bad news....
After fashioning my makeshift coolant return line, I unscrewed the expansion tank cap to top up the system.
When I pulled up on the cap, I noticed that the rubber washer on the very bottom of the cap was hanging off slightly. On closer inspection, I found that the plastic of the expansion tank cap had disintegrated. The plastic lip that holds the last rubber washer on the bottom of the cap had mostly (maybe 80%) fallen off!
Since I'd had that cap off maybe 10 times in the last week or so, I had yet another panic attack: those fragments of cap are likely floating around in the water galleries of my motor!
I immediately removed the expansion tank and flushed it out while being careful to collect anything solid that might be rattling around. What my efforts yielded was saddening: only a tiny fraction of what was missing from the cap was found in my tank.
I can only hope the rest of the cap fragments don't clog a critical gallery, or that the water pump pulverized the chunks before they could do any real damage.
Time will tell.
I have #8 above and a new expansion tank cap on order.
Find attached pictures of the disintegrated cap.
Videos of my situation on YouTube:
YouTube - 2003 BMW X5 expansion tank cap falls apart
YouTube - 2003 BMW X5 expansion tank cap falls apart, pieces recovered from tank
Watch your plastics: BMW did
not make these cooling systems to last...