I'll try to answer some of your questions..
"Do we know if there's service where she was?"
Since he was able to track the phone, there had to have been service in or near the area where the phone was found.
"When they say the phone went "dead" do they mean she turned off the phone, her battery went dead, the service was lost, or she just stopped communicating?"
LE has not given us that specific information. However by logging in to her iCloud account, LE would be able to determine why the connection was lost (phone powered down by person, dead battery, etc) . Also, note that if phone has lost power, the Find my iPhone app does not work. However, if they had her phone set to "Lost Phone Mode", the phone can sense (via button input or extreme motion) if the phone is about to lose connection. And the last location will be stored in her iCloud account. In this senario, you track the phone based on it's last recorded location. My guess is that the phone was thrown > damaged > powered down on it's own.
"KP had to have her Apple password to access the find-my-phone app."
"What if he didn't have that password?"
Correct! Not only was he able to find the phone, but by proving LE with the password he prevented LE from needing to request info from cell phone provider. And Apple will not provide your password to LE- including to the FBI so access to her iCloud would not be possible.
"Would her cell carrier have been able to pinpoint the location of the phone just as easily and speedily?"
No, the cell carrier can only locate the phone based upon the closest tower that the phone last connected (pinged) to. Cell towers are spread miles apart vs Find my iPhone which actually uses GPS for locating the phone.
"Not unheard of for a married couple to share this info but just interesting to me since no one I know has access to my password."
In the case of your disappearance, the person who would be contacted should know your password, IMHO. This could save your life.Hope this helps!!
