You can do it after JEE advance completion. After JEE advance , there will be counselling.

when you are preparing then focus for that. Get a good result and go for counslling. You will get a best college.

No need too worry. There are many students who are giving JEEAdvance and they may not get a good result they can use JEE main results to get an admission.

The organisation look for every way to not disturb the career of candidates!

All the Best for your exam!