Another follow-up:
Since there is no harm in voiding warranty, I figured I'd take the pulley system apart myself for a look.
Taking the belt off was much easier than it was in some older cars. Now that I know, I may make this a regular step in cleaning after rough use. I put an allen wrench into the pinhole to hold the tension pulley open. The little metal loop didn't stand up to the torque of removing the bolt though. Note: the tension pulley bolt is threaded backwards, the other three idlers are normal. All are 14mm.
At this point I spun the alternator pulley, and it feels fine! I doupt the technician ever took the belt off to feel it. I see no reason to replace the alternator at this point.
Two upper pulleys with red seals are the same, and had the most damage. The lower idler pulley has double bearings, and was the only one that felt fine, is spite of being the dirtiest. Perhaps Toyota shoud use this better pulley for all three idler pulleys. The tension pulley has a larger axle. Here's a close-up if anyone wants the part #'s
All the bearings are typical NSK sealed bearings, just like a skateboard.
I carefully popped out the front seal from the inner edge to clean them out. I assume the bearings are pressed into the wheel with much force, so I didn't attempt to pound them out and reach the back seal.
They were flushed and spun in my fingers with solvent, till they spun without any gritty resistance
Then I used a needle on a grease gun to repack them. This pick is posed, it's better to do this inverted so the grease can push any left-over solvent out.
So I put it all back together, switching idlers 1 & 2. It's much quieter now, with the only noise coming from futher away from the alternator. I'm still going to replace them soon, but now I have a little more peace of mind till then. It took longer to prepare this post than to do the job.