How do I subvert this locked uber-secure OSX boot process to do a clean install?
Solution 1:
Apple will typically remove the firmware password as a repair service when you can provide proof of original purchase.
- https://support.apple.com/
Alternatively, you would want the previous owner to send an MDM command or divulge the passcode which they used to lock the device boot order. Your best bet may be to seek a legal remedy if you have a right to own and use the hardware while the previous owner hasn’t performed their duty as part of a sale/transfer of your property.