IIIT

It is very good for you. Many people wants to get this seats in IITS NITS. You are very lucky to have this seat. It is very good option, and don't go for other options. Work hard here and get placed.

I think, both the colleges are good. But Thaper university is much better than IIIT, Sonepat. However, it is private university but placement record is good here and campus is also very good.

NIT-Mizoram is the best option for you at this score.

Course change after the first year is completely dependent on the college and university norms, some university allow it and some won't plus you need to have good scores in the first year to apply for the same. For further details, contact the college authorities.

Hi, please refer the list below:
1. IIIT Hyderabad
2. IIIT Allahabad
3. IIIT Delhi
4. IIIT Bangluru
5. IIIT Bhubaneswar
6. IIIT Gwalior
7. IIIT Trivendram
8. IIIT Jabalpur
9. IIIT Kanchipuram
10. IIIT Kotak.

You can the previous year's category wise cutoff's ranks of NITs and IIITs at:
https://josaa.nic.in/josaaorcrarchieve/Result/OpeningClosingRankArchieveN.aspx?Year=2017
First, check for the Homestate NIT first. There you would get an idea of your selection.

You can compare your rank with previous year's cut-offs by visiting:
https://josaa.nic.in/josaaorcrarchieve/Result/OpeningClosingRankArchieveN.aspx?Year=2017
There, you would get an idea. Best of luck.

MIT Pune is a better option, check out the reviews below for both the colleges for details :
https://www.shiksha.com/college/Maharashtra-institute-of-technology-kothrud-Pune-23583/reviews
https://www.shiksha.com/college/international-institute-of-information-technology-Pune-hinjewadi-47554/reviews.

You should decide for yourself. If you are confident that you can crack the JEE for CSE then you should drop a year, otherwise, don't waste your time. You can also lose what you have now. So, as per my opinion, don't drop and start studying now in IIIT. Placements totally depend on you. If you have good skills and knowledge then you will get placed easily either you are in IIT, NIT, IIIT or any other private college.
So, all the best.
Good luck.