In radar contact?
I understand that radar contact puts the responsibility onto the shoulders of ATC and you're now following a new clearance. Of course with sensible situational awareness on your part, since ATC can make mistakes.
Enroute or before departure, a good scan of the other SIDs and STARs will give you an overview of the area, thus increasing your SA.
Fourth question: if ATC gives a new clearance, and does not restate the previous restrictions, they no longer apply.