I bought my 96 Outback n/a six weeks ago with a slight "exhaust rattle" and the check engine light on. I took it to a good subaru shop, and they told me the CEL was because the rear o2 sensor's heating element was burned out(P0141, I'm guessing). I replaced the sensor with an OEM denso sensor, bought a scan tool, and then kept getting the CEL for P0136 rear o2 sensor circuit malfunction. I checked the wires, and it's all connected, but keeps throwing the CEL every third time I start the car after clearing the code.

While I was down there replacing the o2 sensor, I checked for that rattle. The heatshields felt okay, and nothing seemed misaligned in the system. But when I let off the throttle it makes a hard rattle almost like a truck's engine brake. I dug around some more and noticed that my front cat has been beat in quite a bit, like it hit a big rock. When I tapped on it, it sounds like it's full of loose junk.. which seems like a bad sign.

Last week at a stop light I smelled something like burning plastic or rubber, which I vaguely remember smelling in my dad's car when his cat went out. However, I had recently added a bottle of injector cleaner to the fuel tank, replaced the fuel filter, and changed the gear oil (which drooled 75w90 onto the manifold as it was draining). I have driven about 350 miles since the maintenance, and would guess that anything on the pipe would have burned off, and the injector cleaner would be diluted enough (one tank later) not to matter.

1) Is there any way to tell if my cat is shot without paying for a diagnosis?
2) Would any other suby exhaust systems fit a n/a 96 impreza outback? Could I cannibalize a WRX catback and a legacy manifold? I'd like to keep it quiet sounding.
3) Is the o2 sensor problem related or separate?

Any thoughts at all are GREATLY GREATLY appreciated. Thanks!
Gus