What should I do to save my life?

I understand your frustration of not doing as well as you had expected in your boards. First things first you need to calm down and think on your available options. Assuming that you have the patience to go through four years of engineering first and then two years of M tech, I would recommend that you should take any of the 3 or 4  tier college for now and complete engineering.The reason being once you complete engineering, you have an opportunity to get into any of the IIT's that you want. IISC Bangalore,NIT's,BITS, any damn college that you want. This obviously won't be easy.
The entrance exam that is going to open up so many opportunities is called GATE(Graduate Aptitude Test in Engineering). This is THE entrance test. This test has been taken by many who fail to get  in to the IIT's through JEE.
People claim it is somewhat easier than JEE but underestimating GATE is the last thing you would want to do.Since GATE tests what you have studied through out your engineering, it is important that you take all the courses properly and excel in them.
Colleges in Germany and Singapore also accept GATE scores for admission into post graduate courses. Not to mention, PSU's have 1000's of openings each year for GATE appeared candidates alone. There are many other opportunities that GATE offers and I recommend you talk to a senior regarding the same. In the mean time, go through gate.iitk.ac.in/
and this is just one of the possibilities.
Good luck!
I think life-saving is not a very difficult thing.
If you learn a little, you can easily fight in case of danger.
Please take a look on a course, based on life saving:
Save your life with Krav Maga

Krav Maga is good for men, women, children.. In this course you can learn how to save your life in different life-situations.
I've attached a half-off coupon in this link, hope it helps!
If u dont want to rot, study as much as you can for as long as it is necessary. Study till you get what you want. It's never too late to start. Don't listen to anyone's advice,make your own plan that suit's you.
All the best!

 I too got a under 500 rank with 100% scholarship. It all depends on you. I know someone who is in the same situation. Just dont lose hope keep trying who knows what secrets life has for you. Do what you can so that even if later on you end up doing BA from those colleges you wont think that you never tried cause that will be even worse that studying in that college. If you think you have all the talents that you are mentioning then use them to the fullest and try to make your life a little happier.
Msg me if u need further help
All i can say is,damage has been done ! Now don't spoil your future due to this.Try to study for your upcoming exams,even if you don't feel like.Just give your best considering this can be your last shot.You never know,you may end up scoring more than your expectations in boards.Then you will regret not studying for these entrance exams.Do whatever you can for damage control.Stay positive.ATB.HTH :)
Keep aside your mistakes u have committed till now and drop a year for preparation of bits or mains or advanced,one year is sufficient enough to get into any dream college
Be confident, be determined towards your aim.
This one year can lift up your upcoming future.
Best wishes.
If the goal of your life is to just get an 1 tier or 2 tier college, then i am sorry. Otherwise you need to focus on your life goal and work hard.

And remeber, there are n+1 solutions/options to any problem. N soltuion that you have tried and the +1 solution which will solve the problem. So dont stop trying and dont lose hope. Just work hard towards your goal.
Is it possible to use 6 cell battery instead of 4 cell battery ?

You can, depending on what your requirements are. However, if you need the same voltage, you must drop this extra voltage by some means to suit your circuit.It is even possible that the two sets of batteries will be of different types , with different individual cell voltages, and overall voltage same. In

How and why has Apple overtaken Microsoft?

Apple has not overtaken Microsoft, at least not yet. They don't necessarily compete in the same markets. Apple partners with Microsoft a great deal. Apple has dominated the smartphone and tablet market competing only with Google.Microsoft is still very strong in the ActiveDirectory and small server markets. When I say small server, I mean Windows Server products. Yes,

Will there be a rapid demonetization of the cost of living in the next 20 or so years?

I can see Dr. Diamandis being possibly correct about:transportation (self driving cars becoming transportation as a service and costing 10 to 20 cents a mile instead of 50 to 60 for a car I own). The service life of an electric, self driving car is likely millions miles (World's Highest Mileage Tesla