discharge/overcharge?

Sounds like you should put a piece of tape over the amp gauge and drive it. :D

What does it do if you run about a 10 ga wire from the large stud on the back of the alternator to the positive post on your battery with the car running? (measure voltage at idle and at 1500-2000 rpm)

Besides blowing the fuse your charging system may be doing exactly what they do.
They do have some voltage and amp variations just by design, and 14.5 or so at the high point is fairly normal.
I run a 30 amp auto resetting breaker where you have a 20 amp fuse, and it's possible that the battery needed more than that fuse could handle at the time.