KCET

Yes, you can refer Bosco books. NCERT books are the primary. Later, you can refer Dinesh publications also. It would be better if you understand the concept well and try to solve more questions.

Try to score maximum in theory, you should give more importance to theory. After you feel that you are fine with the theory then you can concentrate on CET.
I feel if you are interested in aeronautics then you better choose mechanical because the base branch for all the aeronautics and aerospace is mechanical. Later, if you are interested then pursue Masters abroad by taking specialization in aeronautics.

Yes, you can take the exam.

Yes, you have to appear for it.

Yes, you can apply for KCET.

Yes, a student having a tattoo write KCET and NEET exam in 2017.

Yes, you have to prepare well. NATA exam is conducted for architecture course.

Kindly visit the link mentioned below:
http://kea.kar.nic.in/cet_2016.htm

Yes, you can. Also, your rank will play a crucial role.