Sims 3 Launch Error

@qtmoosie  It's easy to preserve all your user data during an uninstall, and yes, that would have been part of the instructions.  Saves, saved households and builds, mods and custom content, etc. are all stored in Documents > Electronic Arts > The Sims 3, so you'd just need to make a backup copy of that folder to protect your data.  As a side note, it's a good idea to have a recent backup copy anyway—you never know when you'll need it.

However, since the registry entries are being restored, that explains the issue, and I don't think it's worth uninstalling and reinstalling unless this can't be addressed.  Do you use a third-party antivirus?  If so, it may be detecting the changes you're making to the registry and undoing them.  It's also possible Steam is restoring them itself.

The first question here is whether the entries are restored without your opening Sims 3.  You could delete a couple and leave your computer running while you do other things, then check back in a few hours.  (It shouldn't take that long, but it helps to be thorough.)  If the issue is Steam itself, you could instead launch the game from TS3W.exe without opening Steam at all.  The default location for the .exe is this:

Program Files (x86)\Steam\steamapps\common\The Sims 3\Game\Bin

It isn't necessary to run Sims 3 through Steam after the initial launch.  Aside from the Steam overlay, you wouldn't be missing anything at all.