Engineering Entrance Exam

To speak frankly, you need to study every chapter and you should be perfect in every concept.

No, it not true. There will be always 2 mode of paper, i.e. offline and online.

Probably not but still there are chances. So, you should try it.

Usually the engineering colleges don't give much weightage to the board exam result, their main focus is on the JEE-Mains score but for getting admission in the best college, both of your scores should be good because in today's time, there is a lot of competition.
NIT Calicut, NIT Tiruchirappali, MNIT Allahabad, UPES Dehradun & VIT Vellore are some of the good colleges for pursuing automobile engineering. These accept JEE Mains score but cut off varies from college to college. You can get admission only if your JEE score match their cut offs.

Every student have its own learning and grasping capacity, time table is a person specific thing. You are the best judge of your strengths and weaknesses and the amount of time you need to spend daily. Keeping that into consideration design the time table.

There is no spot round this year in JEE mains.

It depend, in which stream you want to take admission. But, approximately you need 170 marks for top NITs.