The only thing I can answer with? I am not a big fan of bench testing alternators. I have found that not to be 100% concluisve
All you need is the light working, power to the big white wire, and power to the one small wire at plug, the Black/white wire
the other wire is not tested as it really only provides ground for the light to be controlled
You say you have everything needed and it is not charging
I doubt some kind of short
the battery does have to be at least 70% charged in order for the alternator to kick in.
Since you have everything that is normally tested and still no charging? the only things I can say is a porr connection at the alternator plug OR what I lean more to? you do in fact have a bad alternator irregardless if they say it has passed several times
I would charge battery for good while. The try one more time but doubt that is it. Just have to mention in case alternator is sensing just a littl too low battery voltage.
If problem persists? your voltages remain and plug connection is good? then I have to say failed alternator
It really can not be anything else
The diagram is below. It is a simple charge diagram rather than a computer controlled system like most newwer vehicles
I realize you have your doubts about alternator as being bad but,, I am not there, I am going by your tests and voltages and really feel the alternator is in fact failed