Diploma

Hi Parth,
Sorry but you cannot. You can only get admission in IITs after scoring well in the entrance exams such as JEE Mains and Advanced for B.Tech. IIT doesn't admit students based on Diploma programs for Master courses as well. You will need to be an eligible Bachelor with graduation to apply for post-graduation from IITs.
All the best.

HI Shwetha,
No. You cannot pursue MBA without a Bachelor's degree. You will need to pursue your graduation (Bachelor's course) basis your Diploma program or HSC to later be eligible to pursue MBA. You strictly need to be a graduate in order to be eligible for MBA.
All the best.

Surely you can, you can also opt for LEET examination.

Hi,
Different universities have different requirements, not possible to list out every university and college requirements. Generally, top universities require 75% whereas 55% can be the minimum for any tier 2-3 college.