Where have the scientists gone

None that I've found. You always have 3 missions though, so you should always be able to work on something.

All of the Jetpacks handle exactly the same way. However, some of the jetpacks have a flame or other output that can harm scientists on the ground, and some of them are perfectly harmless, which is something to keep in mind for some of the 'don't harm any scientists' missions.

Sometimes you get unlucky with the obstacles, and once the game gets going really fast, a lot of it comes down to luck.

You can purchase the High Mobility gadgets (the Super Sneakers & Gravity Belt) if you what to change the game play physics.