IIT Kanpur - Indian Institute of Technology

First of all, congrats, and you have decided superior colleges.
If interested in computer to for NIT, and if not interested, go for IIT Kanpur. Mechanical and CSE are both the fields of different route. NIT CSE will be good. Moreover, you specified IIIT Allahabad IT, it is again a very good choice. All the options are good. Just see the curriculum and go for your interest.
Good luck.

Generally the cutoff didn't fluctuate and from 3-4 years it wasn't changing .
But no one can say about the gate cutoff for 2018. It will be same or will differ with 1-2 marks.
Most probably it will stay as same .
For choosing any course from particular IIT please ensure that IIT offers the course or not.

For B.Tech from IIT Kanpur, you must qualify JEE Mains and also hold a decent rank in JEE Advanced. Since, you hadn't given JEE Main, I don't think you have any chance of doing that. Also based on your 10+2 marks, you are not eligible for IIT Kanpur, even for NITs and IIITs.

If you are a average student and know your capabilities then you have to work hard, study hard, concentrate and be focus on your studies only and not anything else .
Just go with the concepts of subject of 11th and 12th , practice NCERT as much as you can, Try to solve every question of the book. Go for previous year paper if you want to go for coaching then choose good coaching for the preparation but do not totally depend on it. Do more self study.
You will definitely qualify for 1 attempt in JEE Main.Also if you are of general category then actually you have to study hard.

Both are better but IIT, Bombay gives better placement than IIT, Kanpur. The infrastructure of IIT, Bombay is also better and it is listed in top 10 universities of India.

You cannot get admission in IIT on the basis of JEE Mains, but clearing it is an eligibility criterion for taking JEE Advanced which is the exam for getting into IIT.
For JEE Mains, you need a minimum of 100 marks to stand a chance, but you are still not in the safe zone.
As every year the cut-off revolves around 100.
For Advanced, the max. rank to get IIT-Kanpur or Kharagpur is 30K to 35K but then too you have to compromise with the branch.