Well, today Windows showed a little pop-up offering product support, so I took a chance and followed the links, eventually heading over to the Windows section. There, an agent provided PowerShell commands, and within minutes the problems with Store and Photos, were resolved with the following. It appears there were some syntax errors in the original commands at the beginning of this thread, so I can confirm that these worked:
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Get-AppxPackage -Name "Microsoft.WindowsStore" | Remove-AppxPackage
Get-AppXPackage -allusers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}
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Both Microsoft Store and Photos are now working, problem solved. Thank goodness I didn't need to do another reinstallation.