GPS navigation device wants to work with the data it has and the algorithms and priorites programmed into it. While many have tools to help you force a route (I know Garmin, not RandMcN), once you break that route the device usually must recalculate using its own data and rules.
When I have my own route preference that conflicts with what the programmers of the GPS prefer, what works best for me are paper maps and an orange or pink HiLiter (yellow or green don't work so well for me).
My GPS navigation devices tell me where I am. I don't ask them to tell me where to go.