I'm working on a route too, and this is a question that rised to me some time ago. And my answer is clear: As far as I know, there is no way to set priorities or something like that into bus stops. I have a stop where some buses have only to unload passengers, and others have to load only, but there is no way to solve that. Also, about your other question, I think these 10 passengers are for every bus who stops there, no matter from which route it is. I see that when a bus from my 101 line loads people in one stop and, a couple of minutes later, when 102 arrives (101 is a short trip of 102), there are again these 10 (or whatever) passengers, traveling to any stop, served or not by 101 who passed little time earlier.
I hope any else have more optimistic answers, but that is what i've observed until now.