Unlocking EFI password on iMac

Hello all:

I recently bought a used 2017 iMac 27" 5k at a Estate Sale in Southern California.

I was able to install my own time machine back up from my older iMac via target mode. Everything worked well. after upgrading to Monterey from Big Sur I found that a few of my legacy apps no longer worked as expected, so i choose to boot into recovery and reinstall my big sur back up. after re-booting I was presented a "Locked" lock with a password field. after some research i discovered. i NOW had a EFI password set. After taking it into a Apple store I was told my receipt & paperwork from an Estate Sale was not "technical" proof of ownership and i was refused technical assistance!

So i then called Apple Support and Spoke with a tech named Adrian who only repeated that "technically" i didn't have the proof of ownership they wanted!

this is the case number... Case: ***, Which i can no longer access since talking to "Adrian". Over all I have lost faith in Apple and their lack of support!

What are my options in getting this iMac operational again?

[Personal Information Edited by Moderator]