JEE Main

https://jeemain.nic.in/WebInfo/Handler/FileHandler.ashx?i=File&ii=86&iii=Y
You can download the above link for detailed syllabus.

Negative marking is also there. So, overall, JEE Mains is quite tough than CET, but the ratio of toughness depends on your preparation. MHTCET is focused on testing the breadth of your knowledge and JEE Mains is focused on testing your depth of knowledge; so don't compare their difficulty levels.

In JEE 2019, 12th marks don't matter to it but yes, you have to secure minimum 75% marks in the board exams, only then you would be able to get admission in the NITs and IITs.

I'll prefer you to join any other coaching institute like Allen and Resonance as the coaching institute mentioned by you is also quite good. But the main problem is that their passing ratio is not that good.

Yes, it's hard as JEE Main but not as hard as JEE Advanced. If you want to crack this, you have to clear your all concepts of 11th & 12th standard.

For getting admission in an engineering college, you have to apply for JEE exam. If your JEE Mains score is low, you can get a low NIT, but there are less chance for you to get a good NIT. Apart from this, you can get good government colleges in your area. To get admission in those there is a different counselling form for every state.

Not unique, as there are 4 sets of question paper. In these papers, all the questions remains the same but the sequence changes.

Allen is the best institute for the IIT-JEE preparation, and I'll suggest you to opt for the hostel, if you are residing in home, there is no study environment, and so you can't be able to give your best for your paper.

Colleges have seats for Management Quota also, so you can visit colleges of your interest for discussion, but highly reputed college might not give admission. But, if your ward is serious about doing B.Tech in CSE, then it is not necessary to study in highly reputed college.

For practicing MCQs, I would suggest you to get Dinesh Objective. It offers high level questions that will help you to get through Mains and advanced.
All the best!