UPDATE:
My guy called yesterday to say the unit was ready for pickup. I asked how much. He said $75, i.e. $20 more than I had expected. Said he had to turn the slip ring. Also said this may have killed the reg, but seemed to me to be leaning toward not.
Also said this alternator has definitely had work before. Said the Avalon has more heaters and electronics than most cars of its era and therefore I shouldn't be too surprised that one wouldn't last as long as the one in the old 4Runner.
He did confirm that it was a good ol', 100A Denso.
I picked it up, took it home and reinstalled it.
Here's the interesting part...
I only tightened the belt initially to about a half inch or a little more deflection before I started 'er up. Well... no noise. No vibration with or without the A/C.
Then I backed off the tension, as I did before, to see where the vibration and/or noise would return. And I backed it off. And I backed it off...
Nothing.
I kept going and finally the belt started to squeal like crazy and it slowed way down.
"OH 'SHOOT'!!!" (Rated G version)
Started tightening back up like a mad man and the noise stopped again. (Note to self: Carry an extra belt in the trunk now.)
I shut 'er down and checked the tension. Very sloppy.
So this same belt can now run the A/C and alternator without noise or vibration at a much looser tension than before and quite a bit looser than a half inch deflection. This certainly appears as good evidence that my faith in this mechanic may be well placed.
I tightened it back up to about a half inch and called 'er good for a test drive to work the next day... with charger and extension cord in the trunk still, of course... and no clients scheduled to be in the car.
That was yesterday.
Made it to the office today with no battery light and with A/C on for the entire, whopping, 10 minute commute.
Now something else still...
Not long after I had bought this car, summer came along... and I noted a noise when the A/C came on. I wanted the timing belt done anyway, so I called my mechanic. (Now mind you... this was a 25 year, Toyota master mechanic.) He said it was probably just a loose belt and it would go away when he replaced all the belts as part of the job.
Well... it did.
For quite awhile.
So it appears the belt ASSumption may have caused him to miss the possibility that the alternator needed some attention. I'm sure there are plenty of mechanics with more qualifications than he... who have made worse misdiagnoses, so I'll cut him some slack.
The testing continues...