IIT

JEE prep rank of 2453 for an ST candidate is quite good to get an IIT. Generally to secure admission to newer IITs, candidates typically need to attain a rank higher than 4500 in JEE Advanced. So as you have a good score in your prep exam, you have a good chances of getting an IIT. Although your selection depends on various other factors such as your score in the JEE Advanced exam and your relative ranking within your category. 

Yes, you can apply for the IIT JAM exam, even if you do not have Chemistry as your subsidiary subject in B.Sc. Botany (Hons.). The IIT JAM exam for admission to M.Sc. programs generally has subject-specific eligibility criteria. For example, if you are applying for the M.Sc. in Biological Sciences or other related fields, having a background in Botany should make you eligible, even if Chemistry is not a subsidiary subject.

Some IITs and NITs have offered self-sponsored seat M.Tech Computer Science and Engineering (CSE). IIT Delhi, IIT Bombay and NIT Trichy are some of the prominent institutions that reserve seats for the candidates who are sponsored by their employers. NIT Calicut and NIT Silchar also have self-sponsored admissions. The seats are usually filled through GATE exam but some may not require candidates to take the exam, but they must have work experience in related fields.

For pursuing an M.Sc. in another says top NITs such as Trichy & Rourkela outshine in Chemistry as a newer institute of IIT. Especially, NIT Trichy is one of the reputed institutes for its chemistry program, experienced faculty, and decent placement opportunities. You have AIR 790 in EWS category, the chances of get admission in this established NITs are higher, go ahead.