1. I would not say "she is persistent toward love." Instead, I might say "she doesn't give up," "she does not take no for an answer," or "she is persistent in love."
It is also more natural to say "he rejected her" instead of "he declined her."
2. Hmm, good question. I generally prefer to keep all tenses the same unless absolutely required otherwise. You can use "had" here - "A girl had a crush on a boy, but he rejected her." Then, if you want to say that despite the rejection, she still has a crush on him, you can add: "Yet she still has feelings for him."