"Unfortunately the phone GPS won't acquire a signal in airplane mode. There is no error message "
Because there is no error. GPSes are among the prohibited devices on aircraft, because most of them can and do "leak" some signal. (Probably from something akin to a superhet stage in the receiver.)
I'd be surprised if there isn't a way to shut the cellular radio and allow the GPS to keep running, but cell phones are full of surprises.
no cell phone with a GPS uses a WAAS-enabled GPS, so they will always be less accurate than a "real" commodity grade GPS these days. And on the Verizon system, or any CDMA-based system, they are set up so an incoming voice call will SUSPEND THE DATA SERVICES. If your phone is using the A-GPS modes, that should mean a larger GPS error while the voice call is in progress, too.
WRT the phone chewing up the battery when there is no call signal to be found, that's also normal. On the Palm Treos there are applications that can tell the phone to shut down cellular radio when there is a loss of signal, there's probably something similar for the iPhone since all cell phones work about the same way there: When there's no signal, they ramp up to high power (600mW) and keep trying to ping a tower. When they get a signal, they ramp down to the lowest power that will keep a connection. That's why cell phones today get 5x-10x more standby time than cell phones did a decade ago, they all implement this in similar ways.
Dedicated chartplotters aren't obsolete, yet.