Who most influenced the development of modal logic prior to the nineteenth century - aside from the Aristoteleans, Dialecticals, and Stoics of course?
Many of the thirteenth century scholastic philosophers and logicians developed interesting and original theories of modal logic, based on Aristotle’s account in the Prior Analytics. The analytics was unknown in the West until the middle of the 12th century, and serious work did not begin until the 13th century. Notable commentaries were by Robert Kilwardby (d.1279) and William of Ockham (1288-1347).
Simo Knuuttila’s article in the SEP may be helpful http://plato.stanford.edu/entries/modality-medieval
The arabic philosophers also had theories of modal logic, but I am not an expert.