Should MoUsoCoreWorker.exe preventing sleep be ignored?

I wouldn't be surprised if there is a time limit on how often you can really manually check for updates, once update process is complete any subsequent checks should result in no-op.

If there is indeed such a thing restarting the service would certainly reset that limit as well and would allow you force manual check, but that limit is just my assumption for which I believe it makes sense for a service such as Windows update for obvious reasons, and I think we can even prove that thesis...

If we configured Windows so that MoUsoCoreWorker.exe no longer prevented sleep how are we to know when there is a problem with Windows Update?

I'm not sure how you do this but if you want to track WU operation you can do so in event viewer in the following location:
Application & Services logs -> Microsoft -> Windows -> WindowsUpdateClient

Here go ahead and watch events by refreshing them, you keep checking for updates manually and you'll notice a new message each time telling you basically the same thing over and over again regardless whether something was downloaded/installed or not, I believe this is fair proof of a limit mentioned before.

Why?
Now go ahead and reboot service then check for updates again (manual check ofc.), you may notice the message didn't change, in my case it says that there was 1 update found while none was found, it is referring to 1 update that I installed before doing this test, so I'm fairly certain there is a limit, but could be ofc. wrong.

Regarding you original concern, the powercfg entry seems to imply that computer must not go to sleep until WU completes it's job, if you can't find anything in event viewer (I can't because I don't have this problem) then you can test out the option in WU advanced settings that says:
Restart this device as soon as possible...

I have this option always turned off, so that may the reason why I don't have this issue, but you'll have to test it.