I've never heard about policy settings for Office.
With older Office versions the policy settings were not required for this. But Windows Pro editions support the general Policy Editor for years.
I'm not sure what to do with the .adml files downloaded from MS website.
The download site presents detailed installation instructions. I've just read them and discovered that only "Pro" versions of Office support any policy settings. Thus, my previous advice might be of little use for you.
The site you linked mentions creating documents in older formats (default compatibility mode on file creation), whereas I would just like to convert an existing one. Can you please confirm that this is possible with the linked solution?
No confirmation, but I think that saving a document is independent of its history. Whether you open a previously created document or type a new one from scratch should make no difference at the saving stage.
And can you confirm that thus rendered makefile will actually be acceptable for Trados?
Sorry, no confirmation because I haven't checked that.
BTW, you mentioned possible problems if you install different Office versions in parallel. A solution is a second computer that runs in a virtual machine on the main computer.
(I have Windows 10 Pro, that provides virtual machines out of the box, so I can run my old Windows XP computer in a virtual machine while the hardware was sent for scrap some years ago. My bookkeeping software is 20 years old and needs Windows XP.)