Lateral entry

You can get top college Maharashtra. First, you should have to collect details of colleges which are giving lateral entry. Then, study the admission procedure and apply for that college.

I am not sure in Manipur but in Punjab, you can choose any stream.

First you must have successfully completed 3 year Diploma in engineering. Different institutes consider different entrance exams. Most of the regional, government and private colleges take admission on the basis of State entrance exams.
Entrance exams are:-
Calicut University lateral entry entrance exams.
Delhi Technical University lateral entry entrance exam.
KIIT University lateral exams.
LPU NEST.
UKSEE BTech lateral entry entrance exam.

Yes, you can get admission after qualifying the entrance exam conducted by IPU.

Since you are doing B.Sc through lateral entry system, surely you can take admission in M.Sc in IT by lateral entry system. That would not need the problem. I advise you to just check the colleges like University of Mumbai for this admission. It's a very good college for computer science.

You can consider Jamia, IPU, GNE-Ludhiana, Thapar, SLIET, PEC-Chandigarh, HBTU-Kanpur, IET-Lucknow, MMU-Gorakhpur, etc.

You can consider Jamia, IPU, GNE-Ludhiana, Thapar, SLIET, PEC-Chandigarh, HBTU-Kanpur, IET-Lucknow, MMU-Gorakhpur, etc.

You can fill DTU and Jamia's form. But you have to attend 2 classes in a week but it would be better to do regular B.Tech.