I would like to set up a momentary switch on EXP 1 or 2How can I do this as there does not seem to be any option to select it.

Yeah, I think the freezing is because the MC6 is expecting two other connections (a ring and a tip + ring) and glitches without those connections. That was my initial speculation.

Would an external 3 button Aux switch be shorter to use? Something like this: https://www.americanloopers.com/products/aux-switch-for-morningstar-mc6-three-click-less-buttons

That is, are you saying the MC6 sits too high up to be used as a momentary switch? If so, I think one of those 3 button controllers would sit just as low as a sustain pedal, and then you'd have two additional buttons you could program.

Or if you really wanted to use the sustain pedal, I bet I could make you a custom AUX switch. It would look like the one above, but have two footswitches, a TRS jack, and a TS jack. You would plug the sustain pedal into the TS jack, and wire that up like you would a third footswitch. I am almost certain that would work, so long as the sustain pedal is a momentary, normally open type switch. And I could probably fit that into a 1590a enclosure, so it wouldn't be too much in the way.