Always happy to be in the position to help; you're welcome.
You better be prepared that in Windows path starts with C:\ ...
I think it may be rather personal for what purpose anybody uses save/send and accordingly the needs for filename may be different. For example if screen is small and reader app is showing filenames, there's not much point having every filename like:
Gordon Dahlquist - Miss Temple, Doctor Svenson, and ... disappearing at the edge of the screen.
That said, now I must confess that I use send/save very rarely - for me it's drag'n drop from Calibre folder to SD card.
I keep Book panel open, so Comments field is perfect for me to enter long title for the book. For example -
Simple Smith: How Even a Village Fool Can Have a Fulfilling and Happy Life I enter into Comments with h5 or h6 tags, and
Simple Smith into book title field.
For series I have 'Cycle' field as subseries, but if I needed to save to disk, I'd use just main series name and numeration. Cycle, Awards, Artist etc. info goes into file itself.
OMG, you speak so easily about templates and still hesitate about html ! (In Sigil you just have to click 'Insert picture'. Though I suspect you need something more complicated.)
One more point - when adding retail ebooks (or even most ePubs/mobis) it's rather safe to read metadata from the file itself. Pro side being also, that name of the downloaded file will not matter. I must add some info into custom columns manually anyway, so I'm eyeballing all the other fields too.