1 - Yes, or at least check on the values that the next world inherits from the first one. These values for Ageless Foreign should be checked every time a world is rotated into as a homeworld or becomes one through Change Hometown (if in vacation mode or attending Uni this doesn't matter) as you add new worlds to the mix. It's not an instant emergency if you don't check and adjust each world seconds after arrival unless you typically begin rotational homeworld play at 11:59 pm local time as foreign sims only age by one sim day each at midnight, not constantly throughout the sim day.

2 - I believe that would be all sims regardless of their official residency status.

3 - It makes no difference how Twinbrook is set for Ageless Foreign when Twinbrook is the actively played world. It might matter as to how these settings are exported/imported, but again they should always be checked within each world ongoing anyway.

4 - It depends on what you want the residents of Uni World to do. They won't age while a Uni term is in active progress, so if you want the same students to stay the same age forever then Ageless Foreign should always be on.

What I do is designate one household in one world to be my primary one, and sims in other worlds that I would never play as a homeworld (so for me that's the three WA worlds, Uni, and any other vacation-only worlds) will age only while that one household is being played. The worlds I do intend to play as homeworlds are always on Ageless Foreign. Thus everyone across all of the worlds should age more or less evenly. I'm not actually sure what I do or intend to do with Oasis Landing as its population is a bit more fluid than the others and after trying it a couple of times I'm not really feeling all that engaged with future world play.