It was a couple of consecutive days, then I went on to play another house in the rotation, so I can't say how long it goes one yet.

You don't say what EPs you have.  With FreeTime, there is a chance card that has you bring home a hobby friend after work. In my game too many times, it's been a teen or child (the latest is the paperboy) who should not be at your work place anyway.

Try going into the game's options, click the asterik to display the custom content dialog, then enable CC.  Restart your game.

If you have custom modular stairs, walls, fireplaces, and lights that have custom effects, and installed them without using Scriptorium, then it is possible you installed them incorrectly.  Restore the original files and use Scriptorium to install them.  But if you don't have any of those, Scriptorium won't do a thing for you.

TS2 really doesn't care where a .package file is in your game folders when it comes to mods and objects.  It seems pretty obstinate about having Bodyshop items in Downloads or SavedSims though.  Have you looked at SavedSims at all?

If I were you, I'd look in every folder in My Documents/EA Games/The Sims 2 to see if anything looks amiss.  In addition to the cache files already mentioned, delete Neighborhoods\NeighborhoodManager.package.  Set the Folder view to details so you can see the date associated with it also.  Sometimes the date will provide you the clue you need.

Examine all the folders in Program Files (x86)\EA Games if you haven't reinstalled everything.  I think you said you did, but I may be thinking of someone else having problems.  Again look for any package files that may be amiss, focusing on date, since these files should be fairly consistent on date with some exceptions.  Custom modular stairs are installed in these folders along with one other build mode object, but I don't recall off hand what it is.  Walls maybe?

After doing all that, and you still have problems, the problems reside in custom MESH files as has been suggested multiple times already.

Is there room in the fridge to hold any more?

I'm not surprised.  Bad meshes don't show up until they are finally used, either by displaying it in CAS, Buy or Build mode or some townie was assigned hair or clothing that required the mesh.  And the way they show up is with a game crash.  If the mesh is assigned to a townie, it will pretty much shut down your game until it's removed.  To make matters worse, it could be multiple bad meshes, none of which you know about until the game crashes and you have to go digging again.  You think you found the bad one, but go back into game with everything, and crash again.  Or everything runs fine for a month, and then crash.

Even though you have a 64-bit box, the game is still a 32-bit app.  26GB shouldn't be a problem for your game.

When you have half your CC out of the game, does Clean Installer run?  What about when 25% is out?  If the game and CI work with 50% and then 75% of your CC, I have to wonder if you are hitting the filehandle limit for 32-bit.  What that means is an app can only hold reference to so many files.  If that is the case, purging CC may be the only option, unless you are running multiple themed neighborhoods.

If you have CC that is specific for given neighborhood, such as medieval or fantasy, and all that CC is in your Downloads folder, it's using up a lot of the filehandles and memory.
I made a small program to handle different themes, allowing you keep only the CC in the game that you need for a given neighborhood.  Miros is hosting it at Wooden Simolean.  It may solve your problems if you have themed CC.

The first mod sounds like antifoodnap by Pescado.  If you can't find it listed separately on MATY, it should be in the appropriate director's cut.

NoAutonomousStereo by Christianlov.

If does sound like squinge's mod will conflict with TJ's Visitor Controller.  HCDU will tell you for sure.  In anycase, VC will block strays.  I'd just go with it.

You started having problems after downloading some lots.  In describing what you've done to troubleshoot, you don't say if you uninstalled those new lots.  Looking at your posts, it looks like you don't have all EPs, which opens up the possibility that your new lot requires an EP you don't have.  If you can remember where you got the lots, go back and see if there are any EP requirements.  If you can't remember you'll have to creatively find a way to remove the new lots without removing the lots that do work.  Install date in File Manager might work.

My initial thoughts on reading the error message you are getting in CI is there are too many files for it to process.  If you have your downloads in folders you could remove folders and see if CI starts processing properly.  No matter how much RAM a computer has, a 32-biit Windows program limits memory use to 3 GB.  But then again, it could be the lot requiring a missing EP.

Have you gone into CAS and selected Custom Sim to browse through those?

A little late, but....

The staircases are frequently blocked by a lingering action at the bottom or top of the stairs.  Modular stairs don't have this problem.  The best solution if this happens again is to replace staircases with modular.

The error file you posted shows you are trying to run the Bodyshop that comes with Fun with Pets.  You mention that you've installed some EPs.  What exactly do you have for EP/SPs?

I'm using a standard US keyboard, also with no = sign on the numeric keypad.  But the Enter Key on the keypad functions as an = sign using the calculator, so that may be your key.