2000 Maxima

On my 2000 6 cylinder Maxima, the "Service Engine Soon" (SES) dashboard warning light recently came on and has caused car to be rejected in its' annual Mass state inspection. The fault code readout said "P0430" which I am told means "TWC function, LH Bank". I know that "TWC" means "Three way catalyst".

The odometer shows about 120,000 miles, but the engine starts easily, runs well, doesn't have excessive oil consumption, gets good fuel mileage, has no apparent exhaust leaks or loud noises, and definitely does not smoke.

I cleared the codes yesterday morning. This extinguished the SES light and thereafter I was able to drive the car for a total of perhaps 100 miles (in about 6 short trips) before the light came back on again. I have the following questions:

1) What causes, other than a failed cat converter, can cause the P0430 fault code to be reported?

2) Where can I obtain a service manual that will give specific instructions for diagnosing each of these possible causes?

3) This is a transverse V6 engine. Is the "LH bank" the forward bank or the rear bank?

4) The P0430 fault code translation given above implies that this car actually has two cat converters (one for each of the two cylinder banks). Is this true, and if so is it practical to replace the failed converter without replacing the good one?

5) In a high-mileage car, is it possible and practical to replace a defective converter without also replacing muffler(s) and other downstream exhaust system components?

6) Is it possible and practical to get a used replacement converter?

7) If this is a failed converter, what can cause this failure and what other engine components should be checked to help me forestall the premature failure of a new converter?