Variable output PSU switch configuration
Hmm, I haven't seen someone switch it there before, but it looks like it should almost work.  The only problem that I see is that the 18V setting will try to put that 18V back into the output of the 9V regulator.  I don't think it'll appreciate that.

The 9V setting has potentially the same problem, but I think it's okay because it's less than 18V and the diode from out to in will effectively power the 18V regulator to the point that it doesn't blow up.

Fix the 18V setting, and I think you're good.