I was not actually able to get the infernal wiring connector apart that I was asking about earlier today.
Instead I was able to get enough slack in the wiring harness to bring the old manifold up to the core support. I clamped the manifold to the support, and was then able to get enough leverage on the O2 sensor to get it off with the wiring still connected. Good enough for this job!
Now I need to order a new gasket, and decide if I should bother to try and pry the flange seal off the old one, or just spend another $4 on a new one.
Here’s a close-up of the big ass crack that caused all these shenanigans.
Hopefully this will let me contain what few horsepowers this 993cc beast produces :)