IIT

Hi Harshit,
You may refer to what Sunil Soni advised you here-
https://ask.shiksha.com/am-i-taking-a-risk-of-dropping-1-year-for-cracking-iit-after-i-passed-12th-cbse-board-please-guide-me-in-this-qna-3682543
Good Luck

Depends on ur category

Hi
It would be little difficult to manage both as JEE preparation require regular studies and hard work. If you are very sure of doing something from IITs only, then u may take a gap year and prepare rigorously for it. Otherwise continue with ur b.tech. Alternatively, if you can manage both, u may try again.

if you are thing about cracking iit remember it's very very hard and it's not a child play . to prepare for iit you must first forget about your board exams as they will be an obstacle for your dreams + board will be thrown out of jee mains next year so it won't matter anyway
you need to study at least 10 hours a day and clear each and every dought you have . forget about your friends relative and family it's study should be your religion

Hi
For any engg related diploma, you need to hv sciences as your core subject. Check here for more details.
http://jobsandeducationalerts.com/delhi-iti-admission-2016/

According to me, ML khanna is good for Numericals and RD sharma for theory.
Choose according to your need . If you feel confused on theory part mostly, buy RD sharma. else ML khanna .

Marks-ranks correlation can change wildly from year to year. One of my friend got a 351/480 and was ranked 408 in JEE 2011. In JEE 2010, someone who got 350 was in the top 100.
All I can say is if you want CS in a top IIT, you have to score as high as you can and get a rank as low as you can. For IIT Bombay, that's some 2-digit rank and for other 4 older IITs, it will be some where around 200 or so (in that range). For Guwahati and Roorkee, 500ish or 600ish should be enough I guess.