Mechanical Engineering

You have to choose computer's because along with quantum computing you need to learn some more about the subject where you can find it in the computer's rather than from mechanical. So, I suggest you to opt for computers.
All the best.

You need to score 60 percent in PCM, and JEE exam score is the main criteria to get admission for B.Tech.

I didn't get what you are trying to ask. Please recommend and ask your question I'll try my level best to answer it.

Yes, you can study a PGDM course after Diploma in mechanical engineering. It is a viable choice for many who want to enter the field of management and secure a top job with some of the best companies. Eligibility to secure admission in a PGDM course is mentioned below:
To be eligible for admission to the PGDM programme, the participants must possess a Bachelors degree or equivalent, with a minimum of 50% aggregate marks or equivalent in any discipline, recognised by the Association of Indian Universities for admission to post-graduate programs. Regards.

You need to be specific about your selection of course, according to which you can select the universities and colleges of Canada.

Assuming you belong to general category, you really don't have much options. You won't get anything out of JOSSA, so I would suggest you to explore your other options such as other entrance exams, such as BITSAT VITEE, SRM to get a seat in decent private college. All the best.

It's simple. Just choose CS branch in any undergraduate programme like B.E/B.Tech etc. If you have completed your Diploma, then you can get the lateral entry in UG programme. It is not necessary to have same branch that you have choose in your Diploma.