Figheadair, like many others here, I'm not any good at this game (yet), but I wanted to chime in and say thank you for presenting us with these puzzles. I don't contribute to them often, but I follow them quite closely in the hope of learning a thing or two.

That said, I'll take a stab.

If it was worn at Culloden, it wasn't worn by anyone of great importance; though a well-tailored tartan coat probably wouldn't have been worn by any of the general rabble. With this in mind, I'd guess (based on my impression that the average highlander wore very simple clothing) that it was a coat/doublet worn by the 18th-Century equivalent of a non-commissioned officer - a low-level leader with some means but not enough to be leading a significant number of men.

Would the double-vented back imply that it was intended to be worn by someone while riding a horse?

You keep hinting at the quality of the cloth and the dye, saying that it was home-spun and -dyed... hmmm...

It seems I'll have to keep observing these threads a bit longer.