to answer your last point: In the next releases we've planned to add a summary preview, which appears (optionally ?) right before the user publish a post. Such preview should show how permissions are applied to all the users / circles involved, after having merged them together ( when multiple are selected ).
p.s. We're very aware that circles and boundaries needs a ton of work, to end up with a usable flow that empower the user toward such space of flexibility without overwhelming them, I expect lot of iterations and would be amazing to do several design sprints or anything that would involve community participation for such critical features - any feedback on this is more than welcome!