Hi! Is the following true of all available 2.4 GHz systems (all variants from Sanwa, Ko Propo, Futaba, Spektrum, Traxxas, Flysky, ...)?

In the binding process, transmitter and receiver exchange GUIDs and store them. The receiver is "locked" to the one transmitter afterwards, but the transmitter may have a model memory and be bound to several receivers, communicating with one at a time.

I take it there is nothing stopping one from binding more than one receiver (in one model) to one transmitter. But are there transmitters that can send different signals in that case, so it would be just like having more channels?

Is there any system (or hack) that would allow 2 people with 2 RC models to each keep their own transmitter and switch who controls which model?

If each transmitter has multiple model memory, YES. But you may have to rebind each time you switch controllers,

Bind a different model channel from each transmitter to each car and choose which to operate as you want.

That's the thing, it looks like one would have to rebind every time. Making it impractical in the field.

Or are there any "promiscuous" receivers that don't lock or receivers with "transmitter slots"?

you can go back to 75am/fm controllers/recievers
then just trade crystals

or keep what you have and trade controllers