Diploma

For top universities, you cannot apply. You can get admission in private colleges but you need to clear IELTS with 6.5 band in all sections and not less than 6.
I hope it helps.

Yes, you can.
You can refer to the link mentioned below for institutes:
https://engineering.shiksha.com/diploma-in-engineering-in-bangalore-categorypage-2-55-1-0-0-278-106-2-0-none-1-0

You can visit the below mentioned link:
http://www.bits-pilani.ac.in/dubai/eligibilityCriteria

After completing your Diploma in mechanical engineering, you can opt for CADD as it is the best course. Either you can opt for:
1. Marine.
2. Biomechanics.
3. Piping and
4. Aeronautical engineering.

Yes, you can get admission in B.Sc chemistry after completing your Diploma. But, why don't you opt for B.Sc chemistry directly?

It is either a Diploma or a graduate certificate. Entry requirement for Diploma courses in Canada is 6 bands with not less than 5.5 in any module and class 12th passed.
Entry requirement for graduate certificate in Canada is 6.5 bands with not less than 6 in any module and a 3 years Bachelor degree.

Follow the link and filter colleges according to location :
https://www.shiksha.com/b-tech/ranking/top-automobile-engineering-colleges-in-india/51-2-0-0-0