Samantha Parrington backs up a character, let's say Hellstorm. Hellstorm attacks Crystal Frost. Is there a window of opportunity for Crystal Frost to power up and use her ally ability before Hellstorm readies, or does Hellstorm ready as soon as the attack is declared legal?

Absolutely. When Hellstorm attacks, the effect to ready him goes on the chain. Before it resolves, you can power up a character and trigger her effect. Her effect will resolve before the ready effect, so when he does try to ready, nothing will happen because he is no longer allowed to.