This is a very well written question. I think you have a very good understanding of what you have learned so far, and unfortunately beyond what you have said I do not know of any other hints to help you here.
You are 100% correct about the active forms ending an an
sound and passive ending in an
sound. I can't think of any times this isn't the case.
I think it is actually easier to not think of
as forms of the same verb.
Since the patterns for this situation are not consistent across a variety of verbs it's better to think of these as independent verbs.