Ben,

My “Gulf of Mexico” transmitter and we Dutch engineers can show you how to handle some grease without any reduction of the reliability.(LOL)

When I read post 1 I only can add information about the “Old Radios” I know.

Dual functional sticks already were used (and excepted?) in 1960 during the world championships in Switzerland by OMU and later commonly by most manufacturers in Europe. .
Frits Bosch also did use OMU in 1962 but Telecont 9 in 1963, maybe for the reason the Telecont is triple simultaneous and so he could control aileron, elevator and rudder without any limitation of (triple) parallel input, no delay or hesitation however he did use the system.
Building a triple simultaneous transmitter with Orbit operator keyboard does not make sense so you need at least one double functional stick however you want to use them.
So triple simultaneous (single brain!) transmitters with dual function stick, I think it was the latest development step to proportional. At least Frits did reach second place with it after the proportional.

Picture , my circuit board for dual simultaneous, two tone generators to use on the same moment.
Second picture, as classic pattern flyer I am my own engineer and have all facilities so as in the good old days, the new Old Radio for the Simla in development. 4 channel proportional with black 1,5 mm aluminum casing.

Have fun with flying.

Cees