Having experience with P171, I can tell you it'* 100% NOT the o2 sensor. It is the O2 sensor that is telling the pcm to add fuel due to a lean condition, the PCM is telling you it is adding more fuel that the acceptable programmed parameters. Hence the code.
One fo the diagnostics for P0171 is to check the MAF. You also have a MAF low voltage. I'd head toward the MAF cleaning/replacement, clear codes and see if either MAF or Lean bank come back.
Cylinder 4 misfire. It sounds like you are due for a tune up. Perform the tune up and check the resistance on the posts for the coil labeled 4/1. You should see something in the 6.XXX range. If not..the coil and a tune up are needed.
00 Regal GS: All kinds of cool stuff.