I dont now how many people work on summertime saga but is definetly not one, i meant more on the sense on how they do the updates in wich the dev works on one girl route per update, and how the main story in independent of the girls. However i don't disagree with what you say, it still would be a lot of work for one person, but i think it could be doable if the dev focus on parts instead of trying to move everyting forward at once.
In relation to the girl, wich name i also forgot, i completely agree, but i always felt that up to this point we had only some sort of intro and the choices still didn't matter much.