Diploma

There is no as such qualification needed to become a hacker. The one without any degree but with knowledge can be a hacker as it all about research and hacking mindset. As in professional hacker means as a security engineer. Also if you are from India and if you want to work with government then you need at least certification degree like CEH, CISSP etc.

Yes, she can opt for Diploma courses as Diploma courses can be of one or two years duration or else she can study in distance education mode also (correspondence), it would be a three years course.

Diploma is always 3 year if you have joined after 10th.
But college have 2 year duration if you have joined after 12th.
So, It is possible 3 year mechanical diploma engineering.

yes it's possible you can get job after graduation

You have to crack the entrance exam i.e. JEE Mains.

You can opt for CS, CSE or IT course.

Yes, lateral entry to mechatronics possible after Diploma in mechanical engineering.

If you have completed 10+2 then you can apply for B.Sc and if you don't have 10+2 then you cannot.

No, it's not good after B.Sc non-medical. I think you should try to crack civil service exams.