IIT Kanpur - Indian Institute of Technology

First of all, congratulations brother!
Of course yes. You will get aerospace in any one of the IITs you want because your marks are damn good in Advanced.

Hello, hope you are doing well.
Sorry, but you don't have a chance to get any of the core branches in IIT Kanpur including biological and bioengineering, chemical engineering, chemistry, civil engineering, economics, material science and technology.

No you cannot get admission in IIT by clearing UPTU Exam.

Hello Prince, it's very nice to hear that you have already fixed the aim of becoming an engineer. As a first step, you need to study really well for next two years with understanding the subject being the theme. Must score at least 75% in 12th and thereafter can apply for B.Tech programme in a reputed college.

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.