You can definitely go for diploma in mechanical and then btech in aeronautics! Provided you have to clear CET with good score. Most of the colleges reserve somewhere around 10 percent seats for the lateral entry. So you might have a great chance! 🙂
And yes you can join AME after diploma whose eligibility criteria is -Students who have passed 10+2 Science stream (with Physics, Chemistry and Mathematics subjects) with minimum 50% aggregate marks.
You have to clear class 10th to do a diploma in Aeronautical Engineering so yes you can do it. You can also do a diploma in mechanical depending upon your first choice. After class 12th you can do a B.Tech in Aeronautics indeed. You can join AME too after scoring 50% in PCM Stream. Thank you 🙂