yeah if you do a 2 knob option and they are far enough apart there could be room for the builder to drill another hole in the middle if needed, seems like a good way.   15mm from the top and 15mm across on each side is good for 16mm pots and plenty of room for the middle pot if needed.

for the sockets 20mm from the bottom 20mm across on each side works for me.

for the switch it all depends on what you use, but 40mm from the bottom edge of the case gives clearance for common 3pdt switches and any smaller ones as well.

hope this is useful mike!