> Also, it's my understanding that authentication can be trivially bypassed if the phone is kept powered on? (ie. authentication data is stored in memory? Seems to be the case for at least Touch ID)

It can be bypassed if the phone is kept powered on -and someone is able to start running code on it-. The normal approach of Apple signing a malicious recovery/update image would require rebooting first; there are probably exploits to be found somewhere, but Apple probably doesn't have any intentional mechanism for running code without either rebooting or unlocking the phone (at which point a lot more services open up including developer tools, etc.).

Also, Touch ID expires after a while, and (edit: I said the key is probably wiped, but that makes no sense if the phone continues to display contact names and such on the lock screen; there may be details in Apple's white paper, but since I don't remember any, disregard.)