Well, after all that hard work it seems I've resolved most of the engine issues. After I fired the engine up it had a nice, steady idle but it still seemed to be "hunting" a bit and running rich. I reckon that the O2 sensors are probably heavily contaminated so if cleaning them doesn't improve things, I'll look for good secondhand replacements.
I left the engine to idle for at least 10 minutes and there were no coolant/oil/vacuum leaks. The engine revved up faster when I blipped the throttle and it pulled more strongly during a 10-minute test drive. Performance seems to be back to where it should be. My modified crankcase ventilation system with DIY oil catch can works a treat and the crankcase is no longer being pressurized.
After the test drive there were still slight oil leaks from the front cover and rear crankshaft seal, but they were less than before and the valve cover oil leaks have been resolved. Hopefully the remaining oil leaks will eventually stop now that the crankcase is being properly scavenged by intake manifold vacuum.
I've come a long way since I bought the car and it runs/stops much better than before. I still have a few more issues to deal with but the project is going well so far.