Hi @Syntactic
Thanks for your back and sharing.
Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.", and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
Issue Symptom:
Even save a blank document, the message bar will show 'Security Risk: Microsoft has blocked macros from running because the source of this file is untrusted.'
Current Status:
Solved via adding the top-level editing folder a 'Trusted Location', and all the client subfiles are covered by that.