Harriet's accepting his advice on Wilfred is her saying she's not ultimately going to turn him down and that's what makes him risk proposing, when at the start of that conversation he was only asking leave to go back to the beginning and start again properly. So the chessmen have to be earlier in the thawing process and not as decisive IMO.

As asking for them at all was a gift from her to him, maybe the rules of etiquette have to go into reverse and she gets to keep them, as throwing them back at him would be even more insulting than turning him down.