Polytechnic

Hello,
After diploma, various career opportunities open for you in government sector or private sector. Some companies prefer to recruit Polytechnic students rather than engineering students. About government sector these are the opportunity available.
Indian railway
BHEL
BEL
NTPC
Power grid
Electricity department
Talking about private sector you can get the job at the companies related to your field and with handsome freshers package. If you are lucky enough then you can also work as lab assistant at college. The opportunities are limited provided the person must be skilled.

Yes, you can you have to go through its exam for lateral entry in the 2nd year, if you clear the exam, fulfil its eligibility condition and then you can take admission here.
All the best.

Yes, you can get a free seat with scholarship.

Government polytechnic Mumbai, SH Jondhale Thane, VPMS Polytechnic, Vivekanand Education Society Mumbai, Adhesh Polytechnic College are top college for polytechnic.

In my opinion going for class 11th and class 12th is a good, because you get some more time to explore your options with more insights and experience. If you find Mass communications or Law more interesting, you can have a chance unlike Diploma. If you like after class 11th, if you still want to go for Engineering in CS, prepare well for JEE, state level Entrances, other universities entrance and go ahead.

IITs and NITs do not have provision for lateral entry admission in 2nd year of B.Tech for Diploma holders in engineering from polytechnic.