Even on the grand finale night, are আপনি allowed to vote that night?

Yes. Why would আপনি think different?
They (technically, its just Ryan, but still) give আপনি numbers for the two finalists and then আপনি vote for them. Most of the time they will give আপনি three numbers a person, to keep the lines open. আপনি do 186643657 and then what ever number your vote is.

What do আপনি mean দ্বারা grand finale? The very last show? Because আপনি can't vote then.

i actually dont know but i think আপনি can but only for only when it comes to grand finals of american idol ae