It's been a while since I started this thread and I thought I would update my progress.
It turns out that the registration key I retrieved off the computer was NOT the correct key. I have since found the original disc and reinstalled Windows from a USB drive, after figuring out how to change the system boot order.
Now, Windows 10 20H2 seems to be humming along, nicely.
This, however, leads me to another question. I took up this maintenance project to get the computer up and running for my daughter. In order to do so, I had to set it up under my account. Now that I have done so, I want to wipe the computer and get it back up to the setup so she can set it up herself.
What's my best method to reset the computer to the current version of windows, now that it is installed? Will the computer have to be updated again, after resetting it (which is what I did after reinstalling Windows 10)?
Is there an option of unregistering the PC and reregistering it under my daughter's name?