IIT

Follow link :
https://www.shiksha.com/b-tech/articles/iit-admissions-blogId-12996

It varies from year to year. Also, it depends on the subjects you want. If you want CS in IIT then you need to work really hard and get a very good rank.

Yes, you surely can but you need to refer the JEE preparation textbooks. JEE portion is a little advanced from our CBSE portion. So doubts will surely arise but you can take help of Internet tutorials or guides. Even the students who take special coachings for IIT JEE preparation, self study for better understanding.

If your are B.Tech in Mechanical, Electronics, CSE, Metallurgy, Aeronautical Engineering or M.Sc Physics, Chemistry from IIT you have a very chance of getting a good job with ISRO.

You can appear for JEE main thrice and JEE Advance twice that too in the consecutive years of completing your 12th so you cant appear for JEE after completing B.Tech agriculture

Among your choices, DTU IT is a very good option unless you want IIT brand attached in your CV.