I've played the game few days ago, it was all fine. Now today the game simply restarted its settings. I've tried:

- pasting copied settings [these got rewritten]
- pasting copied settings AND denying writing permissions on the file [game found a way to ignore them even though the file was kept intact - where does it get its garbage settings from?]
- changing settings AND denying writing permissions on the only other file that contains them that I found, the deviceconfig.ini [settings were ignored]
- changing the compatibility mode of TES3.exe to run with Windows XP/SP3 as some forum answer recommended

I want to understand what gives. Where does the game get its settings from if it doesn't read the blocked options.ini file?

Where are you getting these files? The Options.ini file that is read from and written to should be in the TS3 user game folder in Documents. DeviceConfig is a log file (program output), not an .ini file. It gets written to the moment that the game starts up, it doesn't get read from, so changing it in a text editor wouldn't accomplish anything. Could be a misunderstanding on my part, but this doesn't sound like the behavior of the files most of us are used to seeing.

Is it possible that you have a second TS3 user game file someplace, such as within Documents on OneDrive, or in another Windows account's Documents library, that the game is really reading from and writing to at least on and off?

If you've updated your graphics drivers in between the two play sessions the options.ini will reset regardless of what you do. The issue lies within options.ini itself because it contains a reference to the graphics driver in the line Last Device (or something like that). If it finds a driver there that doesn't match your current one it will cause the file to reset to factory defaults.
If you want to keep your old settings you would have to remove your old options.ini, start a game and then copy the numbers in Last Device from the new options.ini, manually paste these into your old options.ini and then replace the new file with the modified old one.

@igazor

From the Electronic Arts/My Documents. Modifying the Options.ini file there in the past was working, but it became really puzzling when I changed stuff in it, blocked it from modifications, and the game managed to ignore it. It must have some 'default' file somewhere else or something. Truly a mystery.

@Don Babilon

Odd thing. Modifying the options.ini file that game 'redid' (overwrote things in) was causing it to just reset all the time. Removing the file entirely so that the game remade it from scratch, on the other hand, allowed me to edit it. Huh. Thanks. I wasn't updating drivers on my own but some silly NVidia game experience updated itself recently. Not that I've ever used the damn thing and now it cost me an hour in Sims 3 settings battle. Thanks, updates, always a joy...

I know the pain; I've manually set the length of all age stages and that of every season to my liking (longer than allowed) and having these reset together with all graphics settings and the lessons after a driver update always drove me up the wall.

So for future references: keep a copy of your edited options.ini in a safe place and if the game resets the settings go into the new options.ini and look for the line "lastdevice = 0;10de;1081;8253" (that is my current one so your numbers may vary). Copy that number and paste it into your old, saved, options.ini, replacing the old number and save that file. Copy the file and replace the one in your The Sims 3 folder with that one. That should do the trick.

That is very good to know. I don't set numbers above what's allowed in game, but I sure to set them to be EVEN. Having some life stage sit at 51 or 67 is just... unplayable.