For those of you with the patience to follow all the trials & tribulations of my partner’s Prizm, I have some good news. Autozone cleared the latest code for her(p0420, Catalyst System Efficiency Below Threshold) and she drove it around for about 80 miles without the light coming back on. So this morning she took it in for her Massachusetts inspection retest(it failed 2 months ago due to the check engine light being on) and it passed! The car is legal in MA again.
She also said it doesn’t seem to be doing the thing where it randomly has no power when the throttle is applied, or she is automatically compensating(if you let off the pedal and try again it worked). So we are no longer desperately looking to replace this heap. Instead she’s going to drive it for a bit and see if it behaves, and if so shes willing to drive it for a while.
Now I might have some time and energy for the issues with my xB & truck.