Forgot pattern for my old tablet; no recovery mode available, ADB key not verified (new computer)
Since the bootloader is unlocked you have a variety of options to pick from and they don't require ADB.
Flash the recovery image to do factory reset
You can use fastboot to flash a compatible recovery image. You can either pick the stock recovery image from official source or choose a custom recovery like TWRP.
When you're in fastboot mode, issue these commands in your console:
fastboot flash recovery FILE    
fastboot reboot

Replace FILE with the location where your downloaded recovery image exists.
Now you can boot into the recovery mode and do the factory reset.
When recovery image still doesn't work
If recovery image still doesn't work for you, issue these commands in fastboot mode to factory reset the device.
fastboot format data
fastboot format cache
fastboot reboot