If I try opening a DXF file by using Windows File Explorer, ( right-click the file / pop up menu Open With -> Inkscape) the system seems to bypass my default template.

Is there a unique template being used?  Or is it opening it with no template?

How can I control this with a template I want to use?

Any help would be appreciated.

I'm not sure how helpful this will be, but I did find .dxf files mentioned in an online Inkscape manual.

Importing Files (free.fr)

The entry said:

"

• .dxf (AutoCAD) Imported via internal script. The import is geared for AutoCAD release 13 and newer files (use QCad to update older files). Note that the imported objects may be very small, use 4 to zoom in on them.

"

Maybe there's more in that manual, that could help you?

The dxf import may be writing its own template, or modifying one in the program files, when opening a dxf from Windows explorer.

I'd ask the developers if there is a workaround: 

Inkscape developers primarily use the Devel mailing list to communicate, so that's the best place to introduce yourself.   But you can also find developers on their chat channel. And the developers who answer your message on the mailing list or chat can give you any further info you might need.