1. The core is exceptionally stable. Some modules crash when you do the wrong thing, or change too much. For using a theme, it should be very stable. Not as much when developing one.
2. Task manager->new task->start LS, or kill it if it is hogging the CPU and locked up. Not serious at all. Do NOT end the process tree when you do this, though, because that will kill most apps started from LS (like everything in your systray).
5. No shell, must use task manager to run a shortcut to set the shell back.
6. Except for some video playback issues (black screen or frozen frame playing back some video formats), yes. If you run into any, search here.