A silly/unconventional idea. What about a curved needle? Would that make fishing layers easier?
I ask, as a beginner in kiltmaking. Trying to get the picture myserlf.
And as to the tread. Would thick (cotton) tread be better, making less damage to the tartan? What about waxing it, like done with leatherworks?
As to the stitches, USAKilts had a recent video, with Barbara Tewksbury, where Rocky and Barb discussed stitching. Iirc they landed at minimum 8-10 stitches an inch.

Just some thoughts, if worth anything...