@97854324 From my understanding, yes, resonance is temporary. It was a way for one sister to let the other see the spirits, but it only worked because the red headed sister was a dormant spirit user herself. Later, she "awakened" and became a regular spirit user like her sister. This is the precedent that there may be two sisters who are both spirit users, one awakening much earlier than the other, but both eventually awakening.
I don't think this means that Cosette has to do the resonance with MC to later trigger her powers, it was just something she did in the past like the sisters. As MC says, she probably would have gotten her powers later on if she hadn't been executed, Cosette or not.
As for her relief, now she knows she really is part of her family, her identity wasn't built on falsehoods, her mother didn't cheat, etc. And, as history repeats, she has evidence that the oracle is not entirely correct about only one spirit user, and she can protect herself.