Unfortunately, I am confident the Disk 1 is dead - I did the usual cable / port juggle (including power cable), booted the machine with just this drive attached (to isolate power delivery problems) etc. It did show up once, early in the process, but only for a short time and only to disappear for good on the following attempts.
The original question remains: is there a way to force Disk 2 from enabled to disabled (without rebuilding) so that I can start the unRaid array and recover Disk 1 using the parity.