Okay, I found something that finally worked for me. Go into the "Call of Duty HQ" folder and delete "cod.exe". Then verify the game files in Steam. It will replace the "cod.exe" file. Boom. That's what fixed it. The executable itself was the issue. Go figure
Just spent 3 hours troubleshooting with every other thread on reddit to get this to work on MW3. This was the solution! Had to comment just to say thanks a bunch!