Julia never grants that the principle behind Marie's actions is correct, even if Marie might not have applied it correctly in this case. In fact, Julia's reliance on an argument by analogy, where all the specific details of the case are changed, shows that her disagreement is with the principle behind Marie's actions, not just with Marie's application of that principle to a specific case.

Julia does not offer additional support for Marie’s conclusion. In fact, she explicitly rejects Marie’s conclusion as nonsense.

This is correct. Julia appeals to a relevantly analogous situation — her analogy of Marie handing Julia the wrong coat — and reaches a conclusion opposite to Marie’s conclusion: she is not entitled to keep what was given to her unintentionally, even if she obtained it without deception, threats, or force.

Julia does not discuss any problems she is facing herself, and certainly doesn't use Marie's criterion to solve any such problems. She strongly disagrees with Marie's criterion.

Julia does not propose any new principles. She only addresses the principle Marie proposes, and explicitly rejects it. And since Julia calls "nonsense" Marie's claim that it was "not morally wrong" of her to keep the extra change, it seems that Julia, far from reserving judgment on Marie's actions, does think it was morally wrong for Marie to act as she did.