Hi r5e, I've been following your steps on the ER605 thread as I've encountered some of the same problems as you did.
I was able to make the full MTD backup, and flash OpenWRT successfully on this unit, and now I want to revert back to Stock Firmware.
I see that this isn't the case with most users in this forum (not many users seem to care to do a roll back), but I see you have shown interest in doing so too.
Did you succeed in your attempts? I've encountered a problem in my scenario, where the mtd3 partition backup is bigger than my initramfs available buffer. This seems to restrict the flash write proccess as wget gets stalled nearing the end of the file transfer using the instruction:
wget -O- http://[url to mtd3.img on a local http server] | ubiformat /dev/mtd3 -S [size of the image in bytes] -y -f -
ubiformat quits with the following error:
ubiformat: error!: file "-" is too large (130809856 bytes)
error 0 (No error information)
I'd love to know your input, thanks in advance!