You can still impress her if you get the timing perfect but make a few mistakes AS WELL AS getting all 100 nobles impressed. If you do everything perfect, it will say, "Out of the 100 nobles watching, 100 were impressed," and then "Queen Brahne was quite impressed," next it will say "They demand an encore! Perform the fight scene again?" Select no and you will be showered with 10,000 gil!
However, I never knew about the Moonstone bit!
You can get one after Conde Petie anyway by collecting all the coloured stones from the mini-alters and putting them all into the holes in the big altar. A moonstone will fall out the back of the altar. That's the point at which add-ons are really necessary anyway.
