Would this letter alone have been enough to alert Catherine to Isabella's real nature? I hope so - Catherine may be a trusting and rather naieve soul, but she isn't stupid.

As for what Isabella means... I think she's just writing whatever comes into her head at that moment. Clearly, she didn't read the letter over before sending it or she would have ripped it up and started again! Even she couldn't have believed that Catherine would swallow so many inconsistancies and contradictions.