"They only plough once a year" I think you will find that that preparation for sowing involves ploughing more that once. Weed control is one reason - plough, wait for the weeds to germinate and then plough again. That's what the ox-ploughman I watched did.

Where ploughboy and milkmaid jobs that younger people did, so making them more likely characters for a romanic tale?

Has anyone ever totted up the relative frequency of the jobs that characters in songs did?