...solo songs from the early Baroque (Henry Purcell) or earlier to the early Classic (Haydn) periods. Indeed even instrumental pieces for strings and winds (think Handel's Messiah or a Bach cantata) would have a figured bass part for the bass instruments and a keyboard or lute.

Any folk song arrangements prior to early classical (Haydn or Mozart) would likely show only voice and figured bass in the sheet music, so keyboard (and I suppose lute) players would be expected to "realize" the symbols on sight just as good modern players can work from sheet music or "lead sheets" showing only a vocal line and chord symbols (see any modern "fake book" for examples).

Student musicians like Mary would likely want to practice these pieces ahead rather than playing them at sight, and since Mary's cooperation was often sought for accompanying dances, I expect her playing was rhythmically accurate and acceptably note-perfect. I might even disagree with Cheryl about Mary's voice-leading (pronounced as in "leading a horse to water"). I think it might have been fairly good, since it was rule-based, and Mary could practice it--and to my mind would practice it, because Mary IMO was a drudge, someone of lesser talent who tries by sheer persistence to compete with those of superior ability.

But while Lizzy (who would also read thorough bass) might miss a few notes or scramble over the voice-leading, Lizzy had a sense of expression and phrasing (of taste, in short) while Mary strikes me as the sort that would plunk down each chord with the same accent, get all the notes right, but lose the sense of the song, much as some people can read aloud satisfactorily but then cannot tell you what they just read without looking back: they can make the sounds but not simultaneously grasp the meaning. In addition, Mary had a weak (possibly strained or whiny) voice.