I've asked the same questions as Riverkilt of two kiltmaking members of this forum, and had conversation with a local professional kiltmaker. And in August I'll have another opportunity to explore these subjects with Barb's co-author, Elsie. Here is my synthesis from all these sources:

Kiltmaking evolved from "do-it-yourself in your cottage" into a craft guild which required a period of apprenticeship to learn the various steps and techniques. As with everything, quality counted in order to retain membership in the guild and, more importantly, a satisfied customer base. The large mills of today require their "stable" of kiltmakers to meet certain quality standards, and are essentially "auditioned" before being employed by submitting samples of their work. If a kiltmaker's work becomes sub-standard, the mills will not send them work.

Without a formal, regulated apprenticing system, how does one learn the trade? (1) Barb told you her path. (2) Another told me he informally "apprenticed" for about a year under retired kiltmakers from a large Scottish firm, but who were re-employed in the USA in other related work. (3) Another told me about flights to Scotland for extended periods of training that consisted of deconstructing other's kilts, sewing one's own kilts and submitting them for examination and critique. He commented that the results of a submission were merely a "pass", or a "fail" without any clue of deficiencies noted. (4) Kathy Lare proudly cites (and rightly so) her credentials regarding training.

Two camps of kiltmakers emerge: those who are self-employed, and those who are employed or contracted by other businesses. The second group must abide by certain standards, or lose their livelihood. These standards include not divulging certain trademarked sources, techniques, specialties, or other "company secrets."

If a kiltmaker retires, and after their non-disclosure clauses lapse, they and the other self-employed may teach as many others as they wish how to make kilts. Those who still need to make a living with kiltmaking may be less inclined to share such knowledge widely, but the part-timer, the hobbyist, the "enthused" (as Riverkilt puts it) can and do share their expertise (thanks again go to Barb amd Elsie.)

My conclusion is that the limiting factor to learning kiltmaking is NOT on the supply side (training opportunities), but on the demand side (how strongly does one want to learn it.) How much time, money, and focus does one wish to invest? That's the question of life: it applies to martial arts, stained glass, musical instruments, building hotrods, seriously collecting anything, learning history, flying airplanes, tying flies, or (_enter your passion here_)... the list is endless.