Diploma

I would advise you to complete your class 12th with good percentage. On that basis, you will be able to get good colleges for your admission.
Study hard and score well.
All the best.

No, you cannot get into NITs through ECET.

For mechanical branch, institutes like YMCA, DCRUST are the best.

You have completed your Diploma. So, for engineering, go through colleges like VJTI, SP College, Vidyalankar Institute of Technology, Thadomal Shahani Engineering College. These are very prestigious colleges in Mumbai.

No, Indian Institute Of Technologies do not offer lateral entry. But, many good colleges offer lateral entry .You can take admission in these colleges after your Diploma.

Greetings from Shiksha.
You have some chance to get admission in Diploma Mechanical Engineering at Sardar Patel College of Engineering in Mumbai. It is good Institute.

Hello Deepa,
You may refer the link below for the list of colleges offering Diploma in aerospace/aeronautical engineering in India :
1) https://www.shiksha.com/engineering/colleges/diploma-aeronautical-engineering-colleges-india

Diploma in Anaesthesia gives good opportunity to practice in corporate and private hospitals. You can even take up ICU. In medical colleges as per MCI rules you can't upgrade yourself beyond Asst professor, hence you get stagnated, where as in corporate sector you can even reach senior consultant post.

Greetings from Shiksha.
Kindly visit this link for Diploma Colleges of Architecture and Interior Designing:
https://www.shiksha.com/design/interior-design/colleges/diploma-courses-pune