Okay, so these might be really stupid questions, but please forgive me. :)
How is the threadfall schedule determined? I mean, how do they know when and where it's going to fall? Were records kept? Or something else?
In Dragonflight F'lar talks about how he'd gone over all the old records marking where threadfall fell and when and he discovered a predictable pattern and made a timetable. That's what they went off of in the 9th pass. If those timetables still exsist who knows, but it's probably something similar.
Yes, they probably had the ancient records of Threadfall even if they didn't believe they'd be used. They'd have been a curiosity than anything. They probably used both that and the last fours turns of experience, along with Starsmith's observations of the Red Planet, in order to make the current charts.