During the notice periods you are not allowed for leaves and if HR want they can increase your notice period depending upon the leaves you are taking during your NP.
Now coming to your questions:-
1- If HR want they can extend your NP
2- They cannot make your NP difficult rather you can do so be confident.
3- Try not to make any chaos and just go to office and do the daily jobs and come back home. You should enjoy your NP rather than getting yourself tangled in the idiotic things.