The ones I used had Tranyet switches, which definitely feel cheap compared to Cherry.
Ponyboy did have a decent deal on some Cherry switches a lil while ago. He may still have some. Look for him at the Buy/Sell/Trade board.
As for...
Could just a single button or switch be mounted on the housing which would connect the (single?) culpurit wire to its own ground or an external ground wire to discharge?
Nope.
If you have both consoles on at the same time, even if only one console is sending power to the controller, the controller would be sending to BOTH consoles.
You don't have to use the box of that db25 switch.
The actual workings of those things is just the switch you see on the front. The switch has 100 prongs circled around it, and they just have wires going to the db25 ports in the back.
I am actually using a 2 device db25 switch in my xbox cab, so that I can easily switch between player 3 & 4 controllers and my light guns. I took the switch out of that little box, and mounted it in my cab. The switch has a friggin ton of prongs, but they are laid out so as to be compatible with a little bit of common sense.