@justwannasim Wow, that's a lot to deal with. I'm sorry you've had such a frustrating experience.
On the off chance that it helps, did you also reinstall the free content that you received for registering your expansion packs? Each one came with some bonus items that aren't always easy to find, even in your purchase history. Here's a list of the sets:
https://answers.ea.com/t5/Technical-Issues-PC/INFO-How-to-Register-The-Sims-3/m-p/1364268#M69455
Otherwise, at this point, tedious as it may be, it's probably best to install your store content from scratch. While 70 items is a lot, it's not as bad if you don't need to redownload them. The original Sims3Packs should still be in Downloads inside whichever Sims 3 user data folder you were using when you reinstalled the content.
First though, please move or rename the Sims 3 folder in Documents\Electronic Arts, so the game will create a new one when you launch it. Start a new save somewhere, doesn't matter where; go into Buy mode, click the button used to filter items by pack, and count your packs. If any is missing, that would explain the missing objects warning, and of course troubleshooting store content would have nothing to do with it.
If this isn't the issue, install one store item in the new folder (again, reusing the older download is fine), load the game, and make sure it shows up. If it does, you can install the reset in batches of around half a dozen items at a time. The launcher doesn't handle large amounts of data very well. Worlds should be handled separately, not in a batch with anything else.
It's a good idea, however tedious, to check your installed content every two or three batches as well. It will also save you a headache in the long run if you make a copy of the Sims 3 folder each time you've confirmed that all the items show up. (The same way you filtered by pack, I think you can also filter by store stuff.) If there's an issue, you'll find it sooner and not have to repeat your work—just trash the new folder, copy the previous one over, and pick up where you left off.
By the way, customer support mentioned Origin, but you didn't specifically say you had an Origin install. If you still have a disc install not connected to Origin, please see this post about missing store content patches. It's not a problem with Origin installs, as the patches are integrated into the game's program files.
https://forums.thesims.com/en_US/discussion/933285/launcher-no-longer-installing-store-updates
If you get everything installed fine, you can copy over a save and test. If you do get the missing objects warning again, make sure all your packs are still loading; sometimes the launcher "forgets" which packs you've checked off (Origin installs only).
And of course the user data folder is structured quite differently in Sims 3 than in Sims 4, not that you didn't know that. Saves are still in Saves, but saved households and builds are in Library, and a bunch of other folders are in different places that have no analogs in TS4. Here's a complete guide to your user data folder:
https://sims3.crinrict.com/en/2011/01/faq-user-files.html
If you do all this and still get a missing objects error, at least you'll know you've done everything right.
P.S. Overwatch wouldn't have anything to do with this mess. You can reinstall it whenever you want, and it won't make any difference.
