There is no difference. ppl usually use 괜찮니?/괜찮아? in general(at least for me) something ending with ~니 sounds kinda sarcastic. I would recommend using 괜찮아?

I hope there would be a day when BTS will come to Athens.

you could use it.. as if you were talking to yourself out loud. but that kind of behavior itself might be unrespectful if the person is very superior. "그분이 누구셨죠?" (if the person you are refering to is superior even then the person u are talking to) "그 사람이 누구셨죠?" (if the preson is lower than the person you are asking the question to)