A work of staggering genius (no but seriously please help me)

Now I'm definitely not saying that I'm NOT a computing genius, but I may have inadvertently made boo-boo.

I've been smoothly running Sequoia on my mid-2012 MBP for over 2 months, first-time OCLP user. Last night, I noticed that the fans were working very hard for how little I was running at the time. I hadn't restarted for a while, so I did to see if that would help.

When I started up again, I noticed that the graphics of the dock had changed - instead of the usual translucent appearance, it was a black background with a white outline. Also the toolbar at the top was black too. I restarted again, but no change.

It was at this point that I had my first moment of genius insight into fixing the problem. I opened the OCLP app and clicked Run post-install root patches. Without checking first whether this was a good idea, I clicked Start Root-patching.

Part way through running this, it completely froze - including mouse etc. After staying like this a long time, I had my second moment of brilliance - I powered the machine off.

When I started her up again, it went through the "Your mac restarted because of a problem, press any key to continue" screen. It then went to the usual start up page of black background, with white progress bar. About a third of the way through, it progressed no further. After some time, I powered off and tried again. At the second attempt, I left it overnight to see if it would eventually progress. It didn't.

Now, fortunately I installed Sequoia as a dual-boot volume alongside my existing Catalina install, so as a worst case scenario I can boot into Catalina, and if there's anything I need since my last backup of Sequoia I can drag it across from there before doing a fresh install of Sequoia. However, I wondered if any of you might be even more genius than me and know if there's likely to be a way to recover my existing Sequoia volume?

Thank you in advance!