Answer 1. You should have sale deed atleast or transfer letter with previous owners NIC that's it.
2. No not necessary. Just the running letter and transfer deed and tax payment page. Transfer deed only if you have not transfered the vehicle to your name. For prevention of theft related scenario better to have it transfered to own name.
3. No not necessary At all. If the police wala is asking actually he is asking for bribe its because he finds you an idiot(easy pray).
4. Never heard about cng insurance again its asked for bribe. If I was ever asked I would personally kick balls of that a hole policeman.
Remember to reply in hard voice and talk like you are the boss. This case only applies when you have the proper registration documents(mentioned above) . NO SUCH DOCUMENTS ARE REQUIRED OF INSURANCE SHYT.