Foxbury is not harder than Britechester. The only thing I noticed was a routing issue with Foxbury because of the stairs outside. You DO NOT want to have a bike if your sim attends Foxbury. The stairs are a nightmare, Britechester does not have stairs so it is not an issue there.
As far as other households you are not playing and rotational play, the rest of the towns will keep progressing, getting older, and yes will die of old age. There are some workarounds for this though. You can either play on a long life span or turn off aging. Personally, I prefer to go with the long life span, this way sims will continue to age, but won't age out of their current life stage unless they were close to doing this anyway.
If you do play rotational, there is an option in the game that once your sim completes a semester, which lasts one week. You can have your sim go to school, for one week, but don't start up the next semester. Don't worry, once they complete their week semester, the game will keep their progress, but they will have to leave the dorm if they don't continue on to the next week.
Now there is also an option to allow your sims to live in the same household, but attend two different colleges, There is a couple of homes that don't have any household living in them already. It is set up between the two college towns.
If you play a sim who is currently in college, you can leave the household to play another family, but be warned that the college students, while in class, will keep progressing in their class sememster.