I need a multiple-switch light switch which can't all be turned off at once
This is possible with a simple guard. The guard should have a notch for each switch that is approximately 4x the width of the switch, except the plastic should not be notched in one of those four positions for each switch. For example, switch 1 should not be notched in position 1, but should be notched in positions 2, 3 and 4.

The guard also needs 4 horizontal slots that correspond to the switchplate screws. The screws hold the guard against the switchplate and the horizontal slots allow the guard to move left and right.

Here's a crude drawing. The red box represents that footprint of the entire switch, the tall black rectangle is the switch lever, and the large black thing with the 4 horizontal slots is the guard. The guard is shown in each of the 4 possible positions.



As you can see, each switch has exactly one position where it can not be turned off. In the first drawing, switch 1 is blocked from being turned off. In the second drawing, switch 2 is blocked from being turned off. And so on.

Let's set up an example:


the guard is in the first position
switch 1 is on
switches 2-4 are off
you want only switch 2 to be on


To move the guard to allow switch 1 to turn off, you must first turn on switch 2 to allow the guard to move. Then you slide the guard one position to the left. Finally, you turn off switch 1.

This could easily be made using snips and a scrap piece of sheet metal or plastic.