I added the graphics card to the .sgr files after the initial attempts to play failed, just in case that was the problem, though I'd have expected an unrecognised graphics card to just come up at the wrong resolution and with low graphics settings. I wanted to try every test I could though.
I then ran repair after the test to revert the game files back to their original state. I did, of course, keep copies of the unedited versions - but repair seemed a cleaner way of putting them back the way they were. I can confirm that the date stamps on the .sgr files reverted to their original date and time after the repair (4 November 2020), so Origin did its job properly.
I forgot to mention it in my original post, but I'd already tried setting Origin to offline mode, and also disabling Origin In Game. I hadn't disabled wifi, but have just tried again after doing that. It still crashes in exactly the same way, I just get an additional "can't find server" on the Welcome page (which is presumably just the Store and the Sims 4 advertising so not an issue).
There's no third party antivirus or cleaner app on my Macs.
Currently testing on the MacBook Air, as the iMac has the 32-bit version installed at the moment so I can play a bit. For now the MB Air has the base game only, so its Intel HD 5000 integrated graphics card should cope. If/when I progress as far as the Sims 3 menu screen I shall switch testing over to the iMac instead. I have all the necessary information to uninstall the 32-bit version and clean up bookmarked for when I need it.
@msarrowette Try playing in a new admin user account on your Mac. Don't sync any data or sign into any other accounts (except for Origin). Don't even use your Apple ID—this should prevent iCloud Drive from interfering.
You can simply sign into Origin and press Play, but if that doesn't help, repair the game and try again. And setting fullscreen to 0 is worth doing again if the game won't initially open; the options.ini file is of course only accessible from the original user account.
New admin account created, The Sims 3.app copied from the original /Users/accountname/Applications folder to the one for the new account since the 64-bit version is user-specific, and then I launched Origin, pressed play, updated when prompted, quit the launcher and then ran repair to validate the game. Then I tried to play.
Alas, I still get the same crash and the same error message.
@msarrowette Are you using a standalone monitor for your laptop, or just the built-in screen? Do you have any third-party software at all that's installed on both systems? I have to admit, I'm struggling to come up with more ideas here, especially since this is happening on two different computers. I'm not saying I'd give up, just that I'm grasping at straws here.
I can also ask if other people have suggestions, but it might take a day for someone to get back to you.
Just the built in screen for the laptop. Keeping it simple.
The iMac normally has a second monitor attached, but I powered that off/unplugged to make sure it wasn't causing the problem. Not sure whether the coding is there to handle multiple monitors or not in the 64-bit version, since I can't get far enough to check; the 32-bit version will launch and play quite happily in full screen mode with a second monitor powered on, but only so far as presenting it as a permanent blank screen. Aspyr's Sims 2 Super Collection handles that better, in that it leaves the screen usable - even though you're going to want to have edge scrolling turned off if you ever click over there, or you fall off the edge of the hood. :-)
As I have a workaround of playing on the 32-bit version, provided I don't go overboard with EPs, I'm in no desperate hurry. Though I'd definitely like to get it working before such time as I'm forced to go to Catalina/Big Sur/next year's macOS. Of course it's possible that Mojave is a factor, though the system requirements state that High Sierra and above are supported.
Sadly there's quite a lot of non-Apple software in common between the two machines, though the majority of that is installed via the Mac App Store, so is sandboxed. The main things that aren't are probably Dropbox, Carbon Copy Cloner and VMware Fusion. And they obviously don't stop the 32-bit version from running.
Thank you for at least trying to grasp at straws on my behalf.
