Hi Zaaphod,
Your issues regarding the activation of Developer Mode in Windows 10 is more complex than what is typically answered in the Microsoft Answers forums. It is better suited for the Web and Software Developer audience on Microsoft Developer Network (MSDN) Forum. Please post your question in the MSDN Development forum. Kindly visit the link provided here: Microsoft Developer Network (MSDN) - Ask a question.
In addition, I have stumbled upon a document in our support site when I was doing research regarding your concern.I thought providing you this document would help you a bit with your concern. The document contains information on how you can properly enable the Developer Mode in your computer, including sub-links, which still leads to the MSDN forum threads for workaround. Anything else that cannot be answered by this document, please feel free to post on the said forum. The link is provided here: Enable your device for development.
Let us know if there is anything else we can do to help.