Which alternator was in when you reversed the cables? You say "about a month ago" for the incident and the replacement. It is possible that you opened one or more diode and reduced the alternator's output to 50 or 33%. Flickering lights at idle would be one of the symptoms. However, if you did damage, there are lots of other electronics in the car that would have suffered too.

If the current alternator (no pun) does not pass load test but charges with no load, I'd put a slipping serpentine belt at the top of my list.

By checking output voltage at both the alternator and battery, you've eliminated voltage drops in all the associated wiring. You have to mind which wiring is involved only when you need to find a voltage drop - which your car does not have. The proper way to be sure is to use the alternator housing (not a bracket) and the output stud as one set of test points, and the battery posts (not clamps) as the other. Difference of less than 0.2 volts under load = no further checking needed. This should be rechecked after the system is restored and the alternator DOES charge under load. You could uncover a loss previously not detected.