If the two memory modules are an exact-match pair (part of a "kit"), it shouldn't hurt to test them both at once. If you have no errors with both, it's highly unlikely you'd get errors with either single stick.
Startup Repair
Before going to a full in-place reinstall (which requires you to already have a relatively stable Windows 10 installed - and yours just took a turn for the worse - why not try a "Start up Repair". It sounds like you can reach your sign-in screen at startup OK, right? If so:
1) While on the sign-in screen,
right-click the
power icon in the lower right hand corner
2) While
holding down your
Shift key, select
Restart
3) From the
Choose An Option screen, select
Troubleshoot
4) From the
Troubleshoot screen, select
Advanced Options
5) From the
Advanced Options screen, select
Startup Repair
If all goes well with the Startup Repair - you can revisit Windows Update, and see if the Cumulative Update still fails (I rather expect it to still have the same problem, and fail). If the update problem is still there, you have the option to try an in-place repair of Windows 10 - which keeps your installed programs and personal data intact. I think your situation is most likely to succeed with the online method (where you basically "upgrade" Windows 10 from your current Windows 10). Here's a link to a guide that shows four ways of performing the in-place repair reinstall of Windows 10 ... you'll be using the online option shown as "Step 7" in the guide:
Repair Install Windows 10 with an In-place Upgrade - Windows 10 Forums
The online method simply means that you visit the Microsoft website to download and install Windows 10 on top of your old installation. You press a
Download button, and then an
Upgrade Now button. How easy is that?
Let us know if things go haywire (or end-up happy-as-can-be ever after)