Starting with Step 4...
"4) Type msconfig in Start Search box, right click System Configuration Utility to Run as Administrator. In tabs for Startup (via Task Manager) and Services after ticking the box to Hide all MS Services"
That seems like a cumbersome way to open Task Manager, but I did it. I took some screenshots to show that there is no checkbox to hide all MS services, but while typing this, I figured out what you meant. In System Configuration, the general tab has options for Startup but we don't touch those. The startup tab opens Task Manager. Just now, I noticed that System Configuration has a separate tab for Services, and that's where there's a checkbox to hide MS services.
That leaves services for a bunch of Dell drivers, my printer, Adobe Acrobat Reader, and some other makers' applications that I use. However, all of these services and applications survive the upgrade-in-place reinstallation from Windows 10 1903 to Windows 10 1903. They are all non-Store applications, and they continue to work. Only Store applications continue to fail.
Now back to Task Manager's Startup tab, they're all associated with my printer and some other makers' applications that I use. Again, all of these survive the upgrade-in-place reinstallation. They are all non-Store applications, and they continue to work. Only Store applications continue to fail.
Looking at the Startup folder by Start - Settings - Applications - Startup, the list is the same as Task Manager displays in its Startup tab, except that this list is almost sorted alphabetically. (In this almost alphabetical list, all lower case letters sort after all upper case letters, i.e. Z and then a. I thought Microsoft hated Unix, but here Microsoft likes the way Unix traditionally did sorting.)
"5) In your browsers' Settings, check Add-Ons or Extensions to disable all but Flash and any Reader helper: How To Disable Plugins/Add-Ons/Extensions. Change the Search to your desired one"
Well besides Flash, I also have Silverlight, a Cisco video codec, uBlock Origin, and Session Manager. (I used the web for 25 years without an ad blocker, but had to install a minimally intrusive one for malicious ads which take over a browser tab and poison the tab's history before displaying a malicious redirected page, which made it impossible to return to the original desired page which sadly had a malicious advertisement in it.) (Session Manager stopped working in Firefox but it still works in Waterfox.) Again, these are all non-Store applications so they continue working after an upgrade-in-place of Windows 10 1903.
"6) Remove any bundled third party security utilities"
Hmm, you want me to remove Malware Bytes. Pardon my suspicion again, but I don't think Malware Bytes prevents Windows 10's installer from installing Store applications (including the Store itself) while Windows 10's installer is doing an upgrade-in-place of Windows 10 1903. I have Malware Bytes installed on several other Windows 10 1903 PCs and their Store works.
"7) Download, install, decline trial version, update, then run full scans with Malwarebytes, the best free on-demand scanner"
Oh, you changed your mind ^_^ But actually my trials expired, and I still don't think that matters.
"8) For problems with apps Troubleshoot problems with an app - Windows Help"
Interestingly enough I can reset apps which don't display properly in the list, and I can reset the Store itself, but these resets didn't help.
Reinstalling doesn't work because Store itself doesn't work. Reregistering doesn't work, as described in my original posting. wsreset doesn't work, as described in my original posting.
"9) Type Maintenance in Start Search box, choose Security and Maintenance, choose Maintenance, then Start Maintenance"
I couldn't find Maintenance that way but only got a troubleshooter for maintenance. Start - Settings didn't find it either. But in Control Panel I found it, and started Maintenance. I wonder if it will show results later.
"10) Type CMD in Start Search, right click Command Prompt to Run As Administrator"
sfc /scannow gives some errors because some applications are in use. For example text input is in use, because Japanese Windows systems would be unusable without it. However, this kind of thing works and continues to work after the in-place-upgrade, because these aren't Store applications.
I've done the dism commands a few times but I'll do them again now to report results.
DISM /Online /Cleanup-Image /CheckHealth
No broken components found. Action completed normally.
DISM /Online /Cleanup-Image /ScanHealth
The component store can be repaired. Action completed normally. (In Japanese, this doesn't make it clear if the repairable thing is the Microsoft Store component, or if it's a place where components get stored.)
Dism /Online /Cleanup-Image /RestoreHealth
Repair action completed normally. Action completed normally.
I look at Start - Settings - Applications and nothing has changed. I think the Dism command lied when it said that repair action completed normally.
I click Start - Microsoft Store and it does not respond. Yes, the restorehealth option is as dismal as the rest of it.
"11) If these still won't repair after running 3 times and performance problems persist I would do a Windows 10 Repair Upgrade which is a simple procedure that"
that doesn't work, which is why we have this thread.
Pardon me while I skip a few.
"20) What we're seeing especially in Windows 10 is that everything depends on the quality of the install so if you want to invest in the very best install of all do this gold-standardClean Install Windows 10 which clears the hard drive to get it cleanest.. You will get and keep the best possible install"
Yes, that works, but it takes a few weeks to reinstall applications and repeat the settings of those applications. That's why I wish there were some other way to get Store and its apps working without deleting my working applications.
Maintenance (step 9) is still running, no result yet.