1. All are possible, but the best are A and D. B is a fairly common phrase to start a statement with: "In the end, the pop group gave a performance." C would be better (but not more correct by rules) if it were until the end
2. Personally, I chose D, but A was also possible. If the reason the band played was to put a finishing note on the event, then for the end seems better. at the end simply means that it happened at that time, whereas for the end means that it happened because it was that time.