you get p0420 if the cat is bad. the o2 sensor in front tells the pcm what the engine is doing and the pcm uses that to adjust fuel.the rear only is there to compare to the front so the pcm can determine the state of the cat.
the catalyst monitor should go ready on your i/m monitor screen when the pcm tests it but you have to drive it a while a few times probably before it tests it.
the fuel trims in ideal world would be 0. but real world the older the vehicle and the state of tune up stuff and all makes it go from maybe 3 or 4 either way 5, or 6 probably acceptable. any more and something is on its way out.
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