It's done!

https://youtu.be/2AIxvOHdLRQ

Fortunately the leaks were all due to the fittings not being tightly seated.  Unfortunately, to fix this I had to undo & redo the compression fittings on each connection.  I also learned that draining the system isn't simply a matter of breaking the loop at the lowest point - water doesn't siphon out of the whole loop.  So every time I disconnected a tube I'd get (distilled) water flowing out.  If / when I want / need to drain the loop I'm going to do it in the bathtub and try to do it with the case upside down!

After the overnight leak test finished I then needed to drain out some of the water so I could add some coolant (which has biocide and anti-corrosion additives).  But I didn't get the connections to the bottom radiator (which I had disconnected to drain the distilled water) tight, so I had to redo those again (but I think I did it better).

On the plus side, the fan noise (even at full power) is much lower (both in volume and pitch) than before.  So I will call that a big win.  I did some stress testing this morning and the CPU topped out at 60C with the fans at 60%.  I need to do some more testing, but I have a sneaking suspicion I'm hitting a thermal clock limiter as the temperature doesn't change much even if I put the fans to 100%.  This might be because I didn't apply the thermal paste correctly so there's air pockets.  And to fix it I'd need to drain the loop completely so I can remove the motherboard.  Ugh!  OTOH, there doesn't seem to be any risk of a meltdown.