If a sneaky household member went and wiped the phone to install those new keys, would the real user not notice that all their data is gone?

The function "replace keys" can have any other programmable feature attached to it. Wiping out all preexisting data, reseting the phone to factory settings, and sending a message to the original owner is all possible and would make any sneaky behavior very un-sneaky. They could even require some form of 2auth or other technique that people use to authorize themselves to banks in order to get the unit-unique key to make the TPM memory rewritable. GPLv3 don't require that installing new keys are easy, but the real owner must have a method to do it.