JEE

Dear Vinay,
No, it's not necessary to give IIT-JEE exam. Earlier T Chanakya, MERI-Mumbai, and MERI-Kolkata were allowing entries through JEE Mains exam only. But in recent times, IMUCET is conducted for the same. IMUCET is must if you are applying for DNS & B.Tech course. You should appear for IMUCET (Indian Maritime University Common Entrance Test) and qualify for admission in maritime institutes.

You'll be pleased to know that Jamia Millia Islamia University accepts scores of JEE Mains examination conducted by CBSE for admission in B.Tech as well as B.Arch programmes. So, yes, you can get admission here on the basis of your JEE Paper 2 marks. While filling the application form for JMI admissions, aspirants need to submit their JEE Main registration number. Candidates are selected based on the score obtained in the entrance test/JEE Mains score and performance in the Personal Interview/ Group Discussion. For more details, you can visit this link:
https://www.shiksha.com/university/jamia-millia-islamia-Delhi-843/admission#

Here, admission is done through JoSAA counselling based on the rank obtained in JEE Mains. NATA score is also valid in here.
According to the stats of JEE Mains 2017 Paper 2, a rank of 2500 in B.Arch paper helps to get a seat in architecture and planing in VNIT-Nagpur. It is a pen and paper based entrance exam. The fee structure is also feasible here. The fee structure is INR 39100 per semester. Yes, it is affordable as there are many scholarships and loans are also provided for the students who are not capable of paying the fee.
I hope that can you clearly estimates the rank for ONB as well in VNIT through above details.

Yes, outside students can apply. But you have to give either JEE or GUJCET exam to take admission in here because admission to BE is done on the basis of rank secured in Gujarat Common Entrance Test GUJCET conducted by Gujarat Secondary and Higher Secondary Education Board (GSHSEB) or Joint Entrance Examination (JEE Mains) conducted by Central Board of Secondary Education (CBSE). 60% of JEE marks and 40% of boards marks are taken into consideration for the ranks.

For JEE Mains, you have to follow the same process through which you appeared for the first time. But when you qualify JEE Mains and Advanced, you need to fill the application form to take admission in IITs or NITs, and at that time they will ask for your HSC marks and that should be greater than 75%. For JEE Mains and Advanced online exam, the application process is same.

Yes, you can get an admission in Chandigarh University for B.E CSE course through your JEE rank, also you need to have an eligibility percentage of 60 (minimum) to secure an admission. All the course details are:
http://www.cuchd.in/uie/computer-science-and-engg/.

In my opinion, better don't waste your time leaving the existing course and again preparing for JEE. If you have that much interest and dedication, prepare in parallel without leaving your present course.

Dropping a year will be well worth if you are determined that you will crack the entrance next time with good marks. Remember, you may regret about not giving entrance exam again only for 1 or 2 years; but there is a risk of decrease in marks next year if you didn't study well or your luck turns wrong. If you want to give a next chance, my opinion will be you should join a good coaching classes focusing admission in IITs or AIIMS. This will hold you set in regular studying. And select one best option, whether to give JEE or NEET.

Yes, it is possible to get admission through Gujarat board and GUJCET and not through JEE.

If you are in general category, then you'll easily require more than 120 marks in JEE Mains.