You don’t want to send the PC message repeatedly. You are effectively reloading the stored preset, and that wipes out your CC message changes.
Eventide would know best about exactly how long an average preset takes to load. But that is going to be algorithm-dependent [huge algos take longer]. If I have to insert a MIDI delay, I usually start at ~100 mS, and adjust it up or down from there.
As for a solution … you could simply program your key / scale changes into successive presets, and PC your way through them. Or, you might use one preset as a base, and send key & scale changes to that one preset [two CC messages]. But I can’t think of a situation where I would send PC & CC messages sequentially with a single switch action.
