TBH i’m not sure about the first question.
I think the frequency is an average of the gap between each vehicle reaching the ‘stop’…if that makes sense…
The second question, the 20min limit is total time from start to finish, not just between stops. If it takes more than 20minutes to get anything anywhere, it won’t go. This goes for passengers, raw materials and goods.
If you want to service more than one town/city I would have a multi-platform station at the source, each for a destination. Then take each line out to the city and add extra trains, making it a double line. The further away the city is the more trains you’d need to bring the frequency down.
Everything is based on the demand for it and how quickly and how often you can move the stuff.