Diploma

Hard to say, education standards are so high in Germany and so is the academic qualification and an applicant's mettle. A diploma will land you nowhere. If Germany is the country you are heading for holding a Masters or a even BS from a reputed university is important. Also the country offers no diploma programs on the whole. However check RWTH Aachen-RWTH International Academy for a short term course in automobile engineering they offer. The program is meant for current undergraduate students who have a strong command of the English language and are interested in automotive engineering.
Regards

It will be ideal if you can do B.Tech as a lateral entry student. If due to personal reasons you can not study further, it is advised that you do AMIE along with job to have better career opportunities in future.

Muzzamil, you will be able to do that as first year syllabus for all engineering streams is same and after first year the syllabus changes and branch specific course start. So, you can take lateral entry in B.Tech 2nd year on basis of your diploma.
Some institutes give admission on basis of LEET exam and some give admission on basis of your marks in diploma.
Some institutes also offer scholarships on basis of your marks in previous qualification.
Check the websites of some institutes for more details.
Good luck.

B.Tech in automobile engineering has limited scope as compared to B.Tech in mechanical because
with Automobile engineering you will only get jobs in vehicles industry where as with mechanical, the whole diaspora of Engineering is wide open for you.
Furthermore, there are not many colleges with amazing infrastructure for teaching Automobile engineering. Automobile engineering is something which should be pursued as a specialization post graduation.

Dear Harshit
Every branch have there own specialty and value.
If you are going for Diploma after 10th and taking CS there then in B.Tech you have to take CS or IT else everything will be ruined. So first make your mind with your interest. Chose what you like then go for it.

It's depend on your selected college in NZ. If there is no criteria about compartment then you will not face any problem.
Contact to college for better clarification .
Best Of Luck :)

You can take Lateral Entry (or admission) in any good college where this facility is provided. Getting admission in a good college depends on your marks.
Best of Luck.