the age of the laptop and the fact you found corrupted system files on it could indicate a failing/faulty hard drive.
dig out the SMART values will help verify that (although they in themselves are no absolute verification).and run a chkdsk c: /r as well.
also, it sounds like you can't 100% rule out some sort of infection/gremlin.
with the hours you have already spent, and the potential of hours remaining, and no guarantee you are going to fix it, if it was mine, I would be backing up my personal data and biting the bullet and doing a fresh install.
it's the only way you know the system (and any gremlins) are not part of the problem.
then from that clean slate, restore your data, reload your printers, Office, emails etc (yep, a pain I know) and hopefully, if the drive is faulty, all the activity will highlight any short comings.
you're still up for a couple of hours work, but the theory is you have eliminated a couple of possibilities.