Or is asking for them essentially saying that she's not, ultimately, going to turn him down?

I don't know that it's a definite commitment, as such, but I don't think Harriet would have felt able to ask for them if she hadn't been willing to accept that this was raising his hopes, so in that sense, yes, it's a sign that she will ultimately accept him.

I'm not sure about returning them - what a complex question of etiquette, fraught with dangers! I don't suppose she would have wanted to keep them if she'd turned Peter down. And it would certainly feel - at least to me - like having accepted a gift under false pretences. Perhaps she could give them to a charity shop :-)