Well, regarding the choices, it is evidednt you are confused because you are filling the JoSAA counseling on basis of previous trends not on the basis of your interest.

Metallurgy and chemical engineering, both are different branches and also, let me tell you both are great if you find interest in them and both are useless, if you find bored with them

So, I would recommend you to discover you niche/interest in a particular engieerning branch, then fill the choices on JoSAA accordingly.

Also, IIT BHU and IIT Dhanbad, both are excellent colleges, but BHU bit better.