Image is done - good!
Hmm..this OS seems to be hopelessly crippled.
In File Explorer, do you see the hidden folder C:\Program Files\WindowsApps? If so, please try the other two commands (if not, can you create it?):
Add-AppxPackage -DisableDevelopmentMode -Register 'C:\Program Files\WindowsApps\Microsoft.WindowsStore_2015.25.15.0_x64__8wekyb3d8bbwe\AppxManifest.XML'
The RED part is PackageFullName. Could be, and I'm not sure, that the PackageFamilyName might work, which would not include the 2015.25.15.0_x64__ and just be Microsoft.WindowsStore_8wekyb3d8bbwe\AppxManifest.XML.
I doubt this will work, and I'm back to the repair install. But we need to be able to mount and run setup.exe from the v1607 ISO. If we can't get that to work, our only other option is a clean install. The user's files could be copied back from the Macrium image when finished, but all programs would need to be reinstalled.