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.