Hi Paparam, and welcome to cruise-chat.
To answer your questions:
(1) If you're certain you want PC, choose it now. Often, there are wait lists for traditional dining, so by changing now, someone can clear the list.
(2 & 3) Typically, a passenger who has traditional dining cannot use the PC dining rooms. BUT, you may choose to dine in the extra-charge restaurants.
(4) Yes, formal night applies to the PC dining rooms, as well.