Diploma

You can take admission in NIFT, NID, Pearl.These are the top colleges for design in India. Admissions are open, you can go to the website of this institute and see the procedure.

Pursue AMIME from Institution of Mechanical Engineers as more optional subjects are available there.

Diploma holders in engineering are not eligible for admission in B.Tech as IITs do not have provision of lateral entry admission in B.Tech. If you have done Diploma after 12th, you can appear for JEE to seek admission in IITs.

Yes, of course you can. If you complete your Diploma with a good percentage or qualify the entrance exam of the college where you want to get admission then there will be no problem in pursuing B.Tech abroad. All the best.

Yes, you can. It will be better if you join B.Tech in aviation or aeronautical engineering as a lateral entry candidate.

Kindly refer to the following link:
https://hospitality.shiksha.com/toursim-aviation-hospitality-colleges-in-bangalore-categorypage-6-1-1-0-0-278-106-2-0-none-1-0

It can be a good start, however to progress further you would have to study degree programs.