If you look at Bochsa’s instructions in his method, from slightly later or contemporary times, he says to play ornaments ON the beat, taking their value from the following note. I am not sure about 6/8 movements, though, but however you play those graces, make them short and snappy.
Those are appogiaturas rather than grace notes in the modern sense. As in Baroque performance practice, they should be played on the beat, take roughly half the value of the “main” note, and often slightly emphasized to make the most of the dissonance and resolution.
For an excellent overview of harp performance practice of the late 18th century, get a copy of Mike Parker’s book “Child of Pure Harmony”. In it, he brings together a lot of information from the many harp method books published during Krumpholtz’s time.