I'm one of William and Thomasine's descendants. Have you been in touch with the Leeds and Grenville Genealogical Society? They're in the basement of the Brockville Museum, and they've got a lot of info. I've been there a few times working on the Giffin side, and I've landed a couple of pretty big hits. The Eyres were apparently a prominent family in the area at the time, so you should find something on them.