you can force the bootloader on any device by wiping the boot partition / sometimes the recovery also.
and that is still an option.
make a backup of the current boot with the psuedo root method and using dd,
then dd if=/dev/zero of=path/to/by-name/boot bs=4096
reboot and you are in the bootloader.
on a linux box, you will need to run sudo fastboot oem unlock
it will only work with sudo unless you have some specific udev rules, which are completely unnecessary and unnecessarily complicated to set up correctly. as long as fastboot is in your path sudo fastboot should work.
doing this on a windows machine just running fastboot when in the bootloader you will more than likely get a "? fastboot" or permissions fault.
i just acquired a zte open c and thought that working with it would help in rooting this phone, but no luck.
a brave soul can try wiping the boot though to get to bootloader and unlock it
in order to get back to the device though you have to reflash the boot.img backup made previously with sudo fastboot flash boot /path/to/boot.img
That sounds pretty dangerous. Wouldn't there just be a verification error anyways since the valet seems to verify the boot.img checksum on every boot? I would try but I don't have the money to get a new phone if it gets bricked and it sounds like It's almost certain to be bricked if this is done.