Polytechnic

Syllabus for Diploma in Food Processing from DEI is-
Cookery larder importance
Catering establishments larder - organisation & layout.
Larder control
Methods of cooking with special application to meat, fish, vegetables, cheese, pulses and egg salads
Classification, balancing of recipes, standardisation of recipes,
Standard yield
Maintaining recipe files,
Fish roux balance, roux blond and roux bruno
Butchery, eggs, poultry and game vegetables
Assembling of cold buffets, sandwiches.
Theory of bread making
Different types of forcemeat and their uses
Pastry cleaning and care of larder equipment
Kitchen stewarding and upkeep of equipment.

Yes, your class 12th marks will affect your placement, but if you want to pursue Polytechnic with lateral entry, then this class 12th marks will matter. And, if you want normal Diploma, then it doesn't matter. And also, sometimes it depends on recruitment board or recruiter.

I think you should study class 11th, class12th and prepare for IIT-JEE.

JMI will be a good choice in my opinion, and as you have secured good rank there you should first go there. Keep IPU as a backup.

It depends on your choice and interests. Always think one more time before taking any decision. Best of luck for the future.

You have to write the ECET exam. It is the entrance exam for engineering through Diploma. By this you can join in engineering. All the best.

There is so many college in Delhi which offers polytechnic in CS.
Polytechnic colleges in Delhi:
Ambedkar Institute of Technology, Delhi
Pusa Institute Of Technology
Guru Tegh Bahadur Polytechnic Institute
Aryabhatt Institute Of Technology
All the best.

You should apply for Polytechnic Diploma in same Trade.

There is so many courses in Delhi for Polytechnic.
Ambedkar Institute of Technology, Delhi
Aditya Institute of Technology
Guru Tegh Bahadur Polytechnic Institute
Polytechnic colleges in Delhi
Aryabhatt Institute of Technology
Pusa Institute of Technology
All the best.