I often use my phone as a GPS navigation system, playing music over my car stereo (streaming data), and charging at the same time. It often cuts out in this setup when it gets too hot so I would imagine that is the problem. Side note: the controller charges the phone over USB?! I don't have mine yet but if this is true this is good to hear!
Yeah I never charge my S4 since I only use it for FPV with the P3. It always remains between 97%-100% because the controller charges it.
caching seems to just happen when loading the maps over wifi. pan, zoom and explore the area you intend to fly and it should be cached. The limitations of the cache are unknown. I wish DJI built in come controls for doing it.
If it's really that easy, that would be great!