So a quick update. After retightening the belts, everything still seems fine except the "full load" test from before. Sitting in drive, brakes on (ebrake too), lights, ac, fan high, rear window defroster, radio, the voltage measured at the battery will slowly drop and settle around 12.3-12.5v at idle. I've sat there for almost 5 minutes to simulate a traffic jam. If I slip it into neutral and rev to about 1000-1100 rom, the voltage jumps back to high 13's. Under low load, it never drops below 13.5-ish and will jump as high as 14.2-ish with higher rpms. So while it appears it may not be charging fully under all possible scenarios, it's not discharging under the worse case. My son has covered almost 200 miles (both local and highway) since the last adjustments and tests. Ran fine with lights and AC on. No battery light. The earlier P1631 has not returned.

When he has a day he doesn't need to drive anywhere, I'll probably pull the alternator and take it to a local parts store to bench test it.