Is board examination will be covered by IIT Jee preparation or to study extra for board?

In my opinion yes, your board's preparation will be mostly covered by your iit jee preparation...as most of the questions comes from ncert so you can easily score above 85% in 12th board...however,you can start preparing from february for the boards and can solve previous years questions to get an idea of how the questions are asked.All the best!

I didnt get your question if it is regarding syllabus it is all the same
Coming to exams boards are all about reading and doing ncert questions for math and phy
Coming to mains u need to put extra effort and u  need to solve various objective books and u need to gain extra knowledge to do to sums