No, you are double dipping on the penalties. Prone character takes a -4 DV against melee attacks and thats all the advantage an attacker gets. Note that being prone actually gives you a +4 bonus to DV against ranged attacks ("take cover!").
In addition the prone character takes a -4 penalty on melee attacks but no penalty on ranged attacks (though as you pointed out you can only make ranged attacks with an x-bow, not bow. You also cannot use thrown weapons except shuriken while prone, since there are no suhriken in Conan I'm not exactly sure what to do with that one... )
Yes you take the -4 penalty to DV on the AoO for standing up because AoO's are resolved, and this is important, immedately before the action that provoked them, so you are still considered "prone" while the AoO is resolved (think of it as being halfway up, on your hands and knees). The upshot of this is that you cannot use the AoO prompted for standing up to trip the character standing up again (or rather you can but it will have no net effect since he is still "prone"). In other words you cannot trip a guy and then use trip to keep him down, the only way to prevent someone from standing up is to grapple them.