jesslla probably refers to the Compressorizer

I never noticed it before AM, but back then my hood only had the premade townies and one or two playables- possibly not enough to cause the issue.
Sure, but now we're talking about Schroedinger's students. If I'm not watching them, are they in school or not? When I click on them, anytime before noonish anyway, it shows that they're definitely not. Perhaps if I just ignored them from 9AM until 2-3PM their progress bars would fill just fine. If that's the case, then Heisenberg provides a more appropriate metaphor, because the very act of seeing if they're in school must boot them out. But in order to be booted out, they must have been in school to begin with and the progress bar shows they clearly are not. However, that's exactly what you would expect and I know that, so the poison must definitely be in the glass in front of me!

I guess I'm off to make more observations (and perhaps get involved in a land war in Asia).

I have to disagree on this point.
I am experiencing school zergswarm clusterfucking and intentionally avoid viewing the school while it's happening.
When I click the portraits of my school-age sims, the dropdown progress bar advances normally once 9AM hits.
Their actual performance bar(in the "job" tab) doesn't actually start moving until around noon, when it finally shows them as "Learning!" instead of "going to school." (They take no school performance hits as long as they make it onto the bus and into the crowd at the gates).
Even though the game should just teleport them into the building if you're not looking - hell, even if you are, I mean, what an arbitrary place to suddenly demand realism- I think it still insists on stuffing them in there one by one.

That's an EA feature, not AM's fault.

I've had a Neurotic Sim autonomously "Bathe until Tranquil." It was the expensive shower/tub with bubble bath and the candle.

I have an experience similar to that of chaos.
If I choose "chat" as an option at the picnic table, they'll chat for a bit while seated, then everybody resets to standing positions around the table and all food and plates vanish.
It probably is normal game behavior, but I doubt that's how it was intended.

@ Grumblesnort
Is it possible that there are books in the Library that aren't available at the Bookstore?

You can delete families from your Library (neighborhood bin), otherwise the game considers them as an option to move in as a "new" family.
If for some reason, you want to keep a copy, backup your Documents\Electronic Arts\Sims3 \Library folder and then delete the contents.

Use "freeRealEstate" in the cheatbox. It allows a family to move into any lot, regardless of price. It has to be re-entered every time you go back to the "edit" screen, though.

It didn't.

What I don't understand is where to find the TXMTs that go with the overcast envcubes. The TXMTs from the base game don't reference envcubes, so those would have to have been overwritten somewhere.

In seasons ->3D -> textures, there are 3 TXTRs called skysphere-overcast-(name)-envcube . Envcubes control reflectivity, so that might have something to do with it.
Also, weather BHAVs and BCONs are in group 0x7F232BABA in Seasons' objects.package. Of particular interest (to me anyway) is the BCON Sky Clarity which lists the chances of cloudiness with no precipitation. They are all pretty low.
I don't know if bumping these numbers up would produce actual clouds as in your "before" screenshots, or if it would produce a blurry haze as seems to be the case with the Seasons sky.
In any event, it's a few more things to check out. I intend to play with those BCON values later tonight.