Harriet's visit with the Martins was during the late summer (August, maybe into September) and that's when he got the walnuts for Harriet. I don't really understand the connection you're making, either to Emma or to MSND with the comment about the walnuts.

I don't think the MSND reference in Emma is meant to set up any sort of ___________ IS ________________ correlation between the novel characters and the charcters in the play---nothing so simple! Many of the book characters 'play' various roles at various times.