So a pretty miserable update on my iMac Pro.
My attempts to get any meaningful cooperation from the seller have all failed. I got a single message from him via the online marketplace, saying he is sorry, can't get the Apple ID and has given me a number to his former colleague who can fix it via some boot hack. I messaged repeatedly, to get even some traction to try and restore it via Apple ID and do this properly - no reply. Since what I had at that point was a brick - I got in touch with the guy he mentioned and arranged to have it bypassed.
The process would limit me to using not the latest OS, for which I don't care and I would also likely lose all my data - but I can manage as most of it is on cloud. There was also always a chance of it getting blocked again, but not that likely and could be bypassed by ust linking it via my other Mac and running a small application. We arranged for pickup, he did his job though it was a tricky bypass because my machine got locked not after a hard reboot, but during the system update. But it got done in the end. Took a long time as he was mostly working on this after hours, as a favour to the seller guy, but fuck it - I waited.
He organised the shipping back, wrapped it in ungodly amount of protective materials and after another long wait during the Irish bank holiday weekend - it finally arrived today. Smashed. Box looks fine, no protective material was removed. Machine screen is smashed. No idea yet if it boots or not, will check later. Wonderful people of UPS have ignored all the FRAGILE warnings.
I'm pretty disheartened at this point. Don't really feel like pursuing this any more, would simply chuck it in the bin and write it off. It's fucking cursed for whatever reason. May as well burn my savings/get a loan and buy a new computer - forget this one ever existed. I feel like I both did nothing wrong throughout and made all possible mistakes at the same time.