As for Kala's recruitment, here's what I'm thinking: one of the comics explained that the Fire Nation was able to kill the nomads who weren't in temples by establishing fake safety areas, using ancient Air Nomad artifacts to make them believe they were legit. Maybe Kala hears about one, not realizing it's fake, goes hoping that she could find the Avatar there, and meets my character and the Avatar there and helps them escape when they realize it's a sham. Thoughts?

Also, since my character did roam for a while before settling down and becoming a nun, it is feasible that she could have known one of the other characters and seeks them out out of desperation.