Thanks so much!
Here is how I applied your advice:
Pedal 1 set as Mono Switch to toggle "Pedal B" on/off, Wah is defined as Kemper's "Pedal B" .
Pedal 1 is connected with the Lehle's 10k jack, TRS.
Pedal 3: Pedal Type 1, Function Wah Pedal.
Connected with the 50k, TRS.
Obviously the Lehle is a Wah-only pedal now, no Volume function when Wah is not activated. It would probably be a mess anyway: when trying to activate/click the Wah with the pedal, the volume level will necessarily change in a completely undesired way.
The (typical) Wah pedal movements activate the switch too often inadvertently, that is my first impression at least. (I'm sure this doesn't happen with the Mission). Using one of the Stage's effect buttons seems the safer method. In which case I would have a closer look at the volume/wah combination again and discard the switch function.
How can/will Volume be disabled when the Wah is deployed? Obviously it should be a "OR" approach, not "AND".