1. Yes. Doing so breaks the tractor.
2. Maybe. The docked units are displaced as a unit. If one of the units cannot be displaced, the combination cannot be displaced. If all units can be displaced, the whole docked unit is displaced together. (5U7d)
3. Rule (5U7d) implies they can be, in which case they will immediately be removed from all limitations of being landed. (Though they still can't move during the turn of displacement, as they are "Stopped".)
4. No, by rule (5U7b).
5. No, by rule (5U7b).