How can I boot back to Mojave from Boot Camp 6 with password-locked boot manager?
I did some research online, and the only ways I found to bypass the firmware password include booting to the recovery partition (which you already tried and it didn’t work) or some trick involving removing and reinstalling the RAM (not an option on your hardware). So here are my suggestions to resolve the issue:

• You said you have no solid proof of ownership. Can you change that?  Assuming the people you bought it from are honest, they hopefully will be willing to send you an official document showing that you purchased the machine and then take that to Apple to have them remove the password. That would probably be the easiest way. Even if they aren’t willing to provide you with the documentation, you might still be able to get help from Apple, but I wouldn’t count on it. 

• If you’re in an area where it’s possible to purchase insurance on mail/shipments, consider asking your old IT admin if he’s willing to remove the password if you ship it to him. Again, assuming these are honest folk, it stands to reason that they would be willing to oblige you. I would strongly suggest offering to reimburse all return costs  (and maybe a little extra for their time) to minimize the impact on their end. 

• In an absolute worst-case scenario, you can continue to use your machine with Windows, and the APFS container will stay exactly as it is.  You will need to purchase another MacBook if you want to run macOS again, but assuming you get the same model you can get a screwdriver that allows you to open the machine and physically move the old SSD into the new machine and regain access to everything*. Or you can purchase software that allows you to mount the APFS container on Windows so you can recover your files if you don’t have a backup. As a starting point, check out the free trial of Paragon APFS for Windows.

*If you’ve already activated Windows, you’ll need to migrate your product key to the new hardware.