In answer to "When does experience in 12 help in learning non-12? When does it hinder?" from Pedagogy
Contributions from people with experience are welcome.
Microtonality shouldn't be thought of as a practice distinct from music making. Experience of learning music will naturally translate into learning music in other tunings. In many cases experience of traditional music may not really be in 12-equal at all. However, the question's framed in terms of a dichotomy, so let's answer it that way.
Mostly it depends on the nature of the experience, and how non-12 is being learned. Making this a useful topic for microtonal pedagogy of course.
If the musician has experience of tonal music with correct spelling, a transition to extended Meantone is natural. Early pieces should enforce the 5-limit diatonic framework and move gradually to more remote notes and harmonies. A theoretical knowledge of harmony also helps because many of those rules were laid down in a meantone context.
If the musician thinks in terms of enharmonic equivalences, a procedure of diving the equal tempered semitone is most productive. This leads first to quartertones (24edo), but also to 72edo, where they can learn 11-limit harmonies.
Musicians with a Pythagorean mindset (most likely string players) may take to schismatic temperament (see Regular Temperaments) or divisions of the semitone (above). They will have much more trouble with extended meantone (also above).
Experience may hinder when it conflicts with the new system being learned. Paradoxically, the most accomplished musicians will have the most difficulty because their habits are the most ingrained.