(1) What's the difference between a .syx and a .h9z file? Don't they end up doing pretty much the same thing, that is install a preset?
Yes, they do the same thing. They contain presets or lists of presets. Syx files are an older format and h9z files are a newer format. When we first release H9 Control, it only supported this newer h9z format, but we added support for the older syx format so that people could import their existing presets.
(2) What happens if one attempts to install a preset for another Eventide stompbox, e.g., attempting to install a preset for Space on Pitchfactor? Does the stompbox firmware recognize this and prevent it?
It depends on how you do it. H9 Control won't let you do this. There are theoretically ways to do, for example if you send a syx file to a device straight over MIDI, but you'd have to work pretty hard to send an incorrect preset to a stompbox. A stompbox would likely just interpret it as being a preset for a different algorithm. If you sent a Space preset to a PitchFactor for example it would think it was a preset for some PitchFactor algorithm, and the preset just wouldn't sound very good.