Diploma

Yes, you can do that. With lateral entry system, you have to write an entrance exam. If you qualify that exam, then you can get direct admission in B.Tech 2nd year.

You can check the below mentioned link:
https://www.shiksha.com/career-in-commercial-flying-all-you-need-to-know-article-13472-1

Dear Writick,
Unfortunately with a diploma, you cannot apply to any German university. Students from India must complete 13 years of education and even if you have a diploma, it won't be accepted. That's how they work. What you can do is enrol into a Bachelor's degree in India, complete the first year. Another option would be to take the IIT JEE. The third alternative is to opt for Studienkolleg in a German university. I will advice you to refer to the websites of DAAD and UNI ASSIST.

You cannot apply for a student visa without an acceptance letter from an university. Talking about your diploma, you must have valid documents proving that you are qualified to apply, a registration certificate won't help. Evidence of funds would also be required.

Yes, you can do that. But, you need to take NATA exam for that.