If this were a normal door lock that someone possessed the master key to, the FBI would have just requested access to the master key temporarily. The FBI is not operating outside the law here: they have a court order. While in some ways it would be more horrific, I'd almost rather see them ask the more analogous question "we would like a copy of your master key" to see how Apple responds, as they'd no longer be able to pretend like it would take them a lot of time to build what the FBI wants: the FBI would just be asking for literally 512 bytes of data that Apple has that stands between them and their goal.

You're really missing the point. Yes the only relevant code that is going to be run on that device will code signed by Apple. I don't think FBI will have direct access to this "key".

The FBI wants Apple to create "malicious" code/update/software version that would allow for multiple decryption attempts among other things. Apple CAN comply with these requests, probably easily. However, by doing so they will destroy trust in Apple signed code and set a precedent.

It doesn't matter WHO has the key because Apple will be acting in proxy.