Diploma

Lateral entry in diploma is only possible in second year, not in the third year and is completely dependent on college and university norms, some university allow it and some won't plus you need to have good scores in first year to apply for the same. For further details contact the college authorities.
Details: https://www.shiksha.com/engineering/course/diploma-in-mechanical-engineering-lateral-entry-chandigarh-college-of-engineering-and-technology-289459

Diploma in Automobile Engineering:
Candidates who have completed 10+2 examination with mathematics and science or an equivalent examination from a recognised board are eligible for Diploma in automobile engineering course.

Yes, you can join SRM, Chennai after Diploma in mechanical. Just go and check the website of SRM and fill application form.

Refer the following link:
https://www.shiksha.com/muser5/UserActivityAMP/getResponseExamAmpPage?examGroupId=366&actionType=exam_download_prep_guide§ionName=samplepapers&fromwhere=exampage&fileNo=1

Hello Vinod,
It's nice to know your desire to study for graduation in engineering. However normally speaking, almost all states allow only students domiciled in the state to seek admission. Hence I suppose you will be required to seek admission only in your state. DTE of Tech EDU of your state will be controlling whole process.

If you want to do it from Frankfinn then you can walk in to the nearest center anytime. They start batches every month.

Hi,
Yes, you can even get a job during the course if you match all the requirements of the job.

Yes, you can get admission in this course after completing 12th.

Yes, it is a good choice as you will be having a Diploma in hand after the course. This will help you to get a good job with a nice salary.