There could be a chance...
So turn off RTH HEIGHT!! Set to 0. If controller turns off or battery low-go-home = phantom roof kiss.
Keep away from walls as it creates a vortex. Keep away from ceilings as it sucks itself into the ceiling. If this happens Do NOT CSC - just down throttle. On a side note.. I think the p3 will damage props if sucked into ceiling. This is to do with angled motors, and hence the props tilted.
I don't think there is a setting in the app to "turn it on".(VPS)
In Pmode the Phantom will use both VPS and gps to hold its position automatically. If there is no sat reception it will just rely on the VPS to maintain its position.