Can‘t really say because I bought KH Spirit cranks (the curved ones that add Q-factor, thereby protecting the knob from being hit unintentionally) precisely to avoid this. With them on my 100mm wide Schlumpf I never hit the button accidentally. With zero Q cranks on my 125mm wide hub (ungeared) of the 36er I hit the cranks regularly with my heels. So, if I had a 125mm wide Schlumpf with zero Q cranks I would prolly accidentally shift quite often. Curved cranks are a good way to go, if you wanna be on the safe side (though only with 100mm hub as the Q-factor would in my opinion become too important with a 125mm hub.
My hub didn‘t have any malfunctions yet, but I think I‘ve done less than 200km on it and only yesterday started to shift on the fly. So I can‘t really make a meaningful comment on your questions 2 and 3.