1. Swappable electric car batteries are the sane solution for fast-charging electric cars. Good to know it's actually on someone's radar. As for the cost, a city bus costs on the order of half a million dollars with operating costs around a quarter of a million dollars a year. With numbers like that, the batteries don't sound all that expensive. How many batteries you would need per bus depends on a number of factors. Charge time is a big one.
2. The trailer would be for mostly highway driving on fixed routes. Not a lot of tight twists and turns. The trailer also wouldn't have to be very long, and it's not as if segmented buses don't already exist. Aside from trailers, there's the possibility of roof mounting, or having some removable seats at the back and putting the extended battery storage inside the bus.