[Jstylezzz]

1st answer: You're taking the error too literally. Foreach is a include, you need to download the foreach include and #include <foreach> somewhere after #include <a_samp>.

2nd answer: This is probably a timer in your script that respawns the cars as soon as you get out of it.

3rd answer: Check some tutorials on creating checkpoints, creating commands, and so on. I think the things (information) you'll need are about sscanf, a command processor and checkpoints.