I have been gifted an e750v2 with suspected bad flash. Not sure what kind of storage is used for the OS on the e750v2...
Does the uboot install-method validate the partition(s) for the OS in any way?
If not, can I convince the e750v2 to boot from a suitable SD-card? (I am well aware of the quality difference between cheap and less cheap SD-cards.)
The E750 does not support booting from the SD card.
If your E750 is within the warranty period, you can contact us via email cs@gl-inet.com to get the replacement.
If the E750 can be entered to uboot, try flashing the official firmware.
I was able to flash the official firmware from uboot. But this did not really answer my question. 
Does the u-boot on E750v2 not have drivers for the SD-card interface? That is, would it not be possible to instruct the original u-boot to chainload whatever is on the SD-card via one or more u-boot variables?
Sorry, I mistakenly thought that uboot could not flash the firmware.
Uboot generally does not have a driver with an SD card interface.
If can, you can look for some information to try debugging the driver SD card and compile the uboot.
Note: Flashing uboot is dangerous, please operate with caution.