If its connected via midi, can I programme the H9 to either sum L&R to a mono out on specific patches (even though there are 2 output jacks) so I can use it as a wet/dry sound and then can I set up to switch the H9 settings to be true stereo on other presets?
No, this is not possible. The H9 cannot be summed to mono on a preset basis. I also do not recommend using an external summing mixer because the best mono signal is connecting Output 1 and leaving 2 disconnected. This way, the algorithms sum to mono as intended.
With a MIDI switcher that is mostly running mono effects, stereo effects should be placed at the end of the signal chain and controlled (turned on/off) via MIDI, since most of these effects tend to be ones that live in an amps effects loop. You’d have to make it a W/D/W setup for true stereo anyways.