Mechanical Engineering

Yes, you can do certificate/ Diploma level courses from universities like IGNOU. All the best.

https://www.shiksha.com/university/Delhi-technological-university/exams/dtu-leet/admit-card
Follow this link.

You are eligible for B.Tech marine engineering as a lateral entry into the 2nd year. Please contact admission team for more details. Contact no.: 02114399500/16/17/08.

Yes, you can. Many countries and colleges will allow you to apply for B.Tech, but some colleges will allow you to apply for advanced diplomas or so. So, you should start checking college's websites for the requirements and email them also.

The definition of best, varies from person to person and is influenced by personal inclinations and ones strength for e. g. some people might to go with regular stream like CS mechanical, Electronics and some might go with not so common branches like Robotics, petroleum, mining. Opt for the engineering branch that you love the most and are most passionate about and analyze the future work conditions, scope and growth prospects before taking admission. You can also opt for psychometric test comprising of Aptitude, personality and career interest to clear confusion.