Can I do B.Tech through correspondence after diploma from Jharkhand?
Please DONTO do any engineering course through correspondence. It is not recognized and will be a waste of all your money. The recent regulations have made it mandatory not to allow engineering courses through correspondence. Rather go for a lateral entry since you are a diploma candidate.
Check details here for Jharkhand diploma to degree
That's a good idea. But if college permits it will be good and according to my view, you'll get more knowledge in b.tech than in diploma better to opt job after btech.