While doing B.Tech from MERI (IMU) this year, can I fill the form of JEE'19 & drop IMU to join NIT (if I get it) next year? Is it possible?

Yes, it's possible but you have to pay concentration on JEE more than on the B.Tech. If you want to get a seat in NIT next year, instead of concentrating on JEE if you do more concentration on that present year, you may not get a seat in NIT. So, there is a chance of losing any one of them even thou