How to bypass DC-DC converter with N-channel high side MOSFET
The trouble with the approach of dual-using the N channel MOSFET from your switching regulator is that the gate needs to be taken probably at least 5 volts higher than the source to switch it on reasonably. 

This is achieved in normal operation because of the bootstrapping (a small amount of switching energy is stolen from the output to produce a power rail that is a few volts higher than your input power rail). The bootstrapping relies on the output regularly switching and charging a capacitor up. Once the output becomes DC, there is no possibility of this idea working.

However, I don't really see that the relay contact couldn't just be placed in parallel with the MOSFET - in effect you short the MOSFET out.