Circuit to select 12v or 5v power source
A solution to keep the circuit simple would be to choose a buck regulator that is able to handle 100% duty cycle. I found two of them with a bit of searching : LTC3864 and NCV8852. There are probably more.

When the input is 12V, it will efficiently drop the voltage down, without heatsinking required. When the input is 5V, it will maintain the PFET on, thus having only the sense resistor, the PFET, and the inductor that will drop some voltage from the DC jack to the powered circuit. You can manage to have a total resistance for this (Rsense+PFET+inductor) in the range of 150mOhm, which would drop 60mV with a 400mA current. This is certainly acceptable.

LTC3864 can be simulated with LTSpice, by the way.

And you shouldn't worry much about the Wifi and switching regulator interference. Keep them as far as possible on the board, and follow closely the recommendations for the layout of the regulator (there are design references for both chips I mentioned), and you'll be OK.