I think I’m getting to old to dedicate my time to a project this big but I have read some of the information in your links and a have written some thoughts below.
Renaming FreeCAD file extensions would be unnecessary but you need to write programs to detect different assembly files.
I think you are over thinking attributes, these are usually determined by each user or company. The main ones you would need to worry about is file ID and location.
IMO a PDM that is setup with projects is too confining because too many files are used in multiple projects. The length of file names are moot, because smaller companies are liable to use names before they move to numbers but a way of checking the file names to insure that they meet the companies standards might be important.
Attributes are normally stored in files and transferred to the PDM when the file is added to PDM, you also need to be concerned about an attribute being changed in PDM and the file being updated to match.
Different companies change revisions for different reasons some change them for fixing typos others don’t. A user changeable workflow is probably more important than deciding things yourselves.
I think PDM should be separate from FreeCAD. It may require a separate PDM for each type of assembly file.
Good Luck.
Dan