I took the Windows 8.1 Pro ISO, mounted and ran it.
I wonder if instead of running it you could have used it as a /source: for DISM? I still think we (someone) have to figure out how to use that technique. I think it might have avoided your (re-)activation problems for example.
Glad to see that you have a solution. Thanks for sharing it.
I have an 8.1 ISO, except fully expanded. I can't run setup because it asks me for a Windows 8.1 key and will not accept my 8 key. I've tried various dism options. Nothing happened, but I was never confident that what I ran was correct. If you'd like to make a suggestion as far as what I should run on the command line I'd be happy to try it. The 8.1 files live at H:\Windows81\ (e.g., that's where the setup.exe file is located).