Since the engineering colleges have mushroomed in almost all states like Karnataka, Tamil Nadu, Orissa, West Bengal etc, there would not be any difficulty in admission in such private colleges. Even scoring a 40 percent would suffice the purpose of admission. In the event of getting an engineering degree from such colleges, you are not likely to make any headway with respect to your career in the engineering field.
However, the other rout for admission in engineering colleges would be to face the competitive test conducted the different state governments. Being domiciled of a particular state, you have to appear for the competitive test of your own state and on the basis of your performance on such test, you may grab a seat in the state governed engineering college. Here you would get an opportunity of campus placement from different employers. You may consider this option.
However, IIT tag has a different meaning and your sustained effort to clear the IIT test would change your career graph for good. So, you should make a concerted effort for cracking this test.
Yes, by passing twelve, you may take admission in engineering diploma. On passing the same with distinction marks, you may claim admission in the second year of the engineering degree course. In the government engineering college, you may have to clear the written test followed by the oral interview.