MNNIT Allahabad - Motilal Nehru National Institute of Technology

Frankly speaking, you are not having any chance to get admission into MNNIT as for admission in any NIT you must have cleared JEE Mains.

It depends on your category. If you were in general category, then it is not possible to get CSE in MNIT.

No, this is a nit college and admissions are done on the basis of JEE Mains rank . You have to qualify JEE Mains to get admission. Class 12th marks are only for eligibility not for admission chances in NIT.

You can check placement stats here:
http://www.mnnit.ac.in/tnp/placement/placementarchive.php
http://www.nitkkr.ac.in/sub_courses.php?id=371&id3=115
and I think MNNIT is the best, in terms of placement as well as learning.

I think not but you can still do counselling so you can get an idea. I will suggest you drop and prepare if you are interested in engineering.

Both are good and I think you can choose according to your location and comfortability. For your reference the placement stats link is:
http://www.mnnit.ac.in/tnp/placement/placementarchive.php

You are in general category, so your chance is very low.

If you are getting chemical at old iits I would say go for it. Old IITs (Delhi, Mumbai, Chennai, Kanpur, Roorkee, Kharagpur, Guwahati), else compare each IIT placement and MNNIT placement then choose accordingly.

Everything depends on category, and the cutoff which varies from year to year. For general category, 15k rank may be the last at which you can get admission in MNNIT.
For mechanical engineering, it can be 7k approx

Don't think so.
Let's look with different aspects MNNIT is top 5 in NIT, placement stats 94% approximately, MHRD ranking 48, highest package 45 lakh and as a student of MNNIT I can say coding culture is great in MNNIT.
While in IIIT highest package is 39 lakh but I believe that there are less opportunities in IIIT as compared to MNNIT.
Not only placement matters, extracurricular activities also plays a great role in the development of the students.
We have Tech fest (avishkar), culrav, Renaissance, athletic meet and many more things to keep you busy and make you feel alive.