Hello there! Going by topics:
- Yes, it is possible to beat her on a race, but it was made to be hard on purpose. By beating her Isa can gain extra agility points, that's why it is so hard.
- Her fight, however, wasn't supposed to be like that... Her fight really is really supposed to a hard fight as a turning point at the story itself (even the previous White Pieces didn't really want to defeat the party, but she have no problem in that), but there is no such mechanic as her ignoring turn orders like that. It might be a really rare bug (if that's the case, I don't even know how to fix it without seeing the backlog on your game, which is impossible). It can also be that she's using an Ice attack that's freezing the girls and they're losing their turns because of that. I'll look more into this fight today, but the best recomendation I have is trying to change your strategy to fight her for the moment, like trying other Crystals or equipment.