"Wet humidity" is zero problem for either clay or lime plaster. Rain, on the other hand, is. As others have noted, overhangs are critical. My SB in the BC Interior is clay, and vulnerable to rain but protected from vertical rain with 4' overhangs and from horizontal rains with landscaping. (It's also passive solar so the overhangs are built for the latitude.)
The suggestion of Larson trusses is good, but you might get better insulative continuity from something like lime-based hempcrete or papercrete which can be made very lightweight with perlite (so no need for weight-bearing foundation). Breathability would be a consideration but if the inside is painted I'm not sure I'd worry too much-- permeability requirements depend on whether you have more heating hours or cooling hours. If more heating hours, paint's impermeability is good. If more cooling hours, it's not.
You have a lot of questions but not enough info posted to answer them adequately. The one answer I can say unequivocally is don't use SB without plastering it-- it is definitely neither fire resistant nor rat resistant without plaster.