Alternator output at battery (14.1v) when idling with no load, with A/C and lights on (12.7v)

I was picking my daughter up this afternoon from a friends house and left the Galant idling in their driveway for about 15-20 minutes with the a/c on. When I placed the shifter in reverse the radio and cell phone charger (in cig lighter) both shut off. At the same time, the idle rpm dropped but it still was running. I was really confused. I then turned off the ignition and removed the key, waited about 10 seconds and tried to start it hoping the problem would magically disappear. It didn't. I tried to start it up and the engine turned over once; that was it. I was able to jump start it, drove it back home and started researching the cause and possible solution.

I have a multimeter and tested the battery with everything off. It was 12.8v.
idle no load - 14.1v
idle with a/c and headlights - 12.9v then dropped to 12.8v, then 12.7v (over a minute or two time span).

What's most likely the problem?

This may or may not be useful or relevant to the current problem:
This past winter I found the locking bolt on the alternator had backed out causing the belt to loosen and squeal like a dying animal. I put some tension back on the belt and made sure it the locking bolt was secure and the squeaking stopped.