for the first two, i would think your vent valve is stuck open, you can take it off and test it. its sort of by the fuel filler on the inside of the fender. you can take the filter off and blow through it, if you apply battery voltage one way you will be able to blow through and the other it should seal. they usually hang up visually if the solenoid is still functioning you can squirt some wd40 in it and actuate it a few times and put it back in.
the last two could be circuit issues with the sensor you have access to drawings for the rear o2? you should check for voltages at the plug and the wiring to the pcm, if those check out it may be the sensor
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock