Possible to Solder top of through-hole header?
You shouldn't need to solder on both sides of the board (if I understand what you are asking). Your holes should be plated through (are they?) and therefore they always "act like a via", regardless of whether you solder anything in there or not.

Also, you do know that vias can be really teeny tiny, right? If you're concerned about space efficiency, surface mount parts are your friend.