you know...i had those exact questions a couple of years ago and i hit up on my state's extension services (state of new hampshire)....'cept i wanted to know if i could plant edibles atop the leach field, and here is what i was told:
elderberry bushes (Sambuca) are the best edibles to plant atop the leach field cuz its roots don't go deep so none of the waste liquids will go up the roots and affect the plant. they also recommended sarvisberry (Amelanchier) and currants, but i settled for the elderberries.
if you plant ANY kind of evergreen, but especially tams, you are giving an open invitation to spidermites to come into your house and infest your house plants. spider mites can come into ANY house, even if it's sealed, double windows, etc., etc. don't take the chance.