Quick question. Will this mono/stereo switch system work on my mixer panel?
Your scheme is overly complex. First of all, the left input is always connected to the left output, regardless of the switch position, so you might as well just connect them directly.

And there's no reason not to leave the right input connected to the right output even if you're not using it, so just wire that through, too.

So the whole thing boils down to a single SPST switch that shorts the left and right channels together.

However, this might be a bad thing if whatever's connected to Lin and Rin have low source impedances, so it might be a good idea to include some series resistance between each of those jacks and the switch. The value of resistance required would depend on the load impedances connected to the output jacks, which you haven't mentioned.

An even better approach would be to add some unity-gain buffers between the switch and the output jacks. Then the resistance used for mixing won't depend on the load impedance.

The whole thing ends up looking like this:







simulate this circuit &ndash; Schematic created using CircuitLab