I have 84 percentile in Jee main Jan and 67% in CBSE. I can use OBC certificate in central clg but not in state. What is the best I can get?

Yes, you can participate in the CSAB special rounds even after accepting a JoSAA seat in the NIT+ system, provided you follow the specific rules. 

No, you need to fill fresh choices for CSAB rounds. The choices filled in JoSAA will not be carried to the CSAB counselling.

Candidates can check the JoSAA cutoff on the official website - josaa.nic.in.

Yes, the JEE Main admit card is needed at the time of JoSAA counselling.

Yes. You can. Both counselling have different purposes. JoSAA is for admission to IIT and NIT+ system, while JAC Chandigarh is for admission to  Dr S. Bhatnagar University Institute of Chemical Engineering and Technology; Panjab University, Chandigarh (Formerly Department of Chemical Engineerin