They aren't asking for that... they just want in to this particular phone. In fact, wouldn't someone have to unlock this phone BEFORE being able to do an IOS update to install some back-door software?
No. The one and only reason Apple can even attempt what the FBI is asking them is because you *can* load a new version of the firmware without a PIN when the phone is in DFU mode. Its a last ditch effort to rescue a bricked phone. If that didn’t exist they would have told the judge ‘sorry...cant help you’.
Again, what the FBI needs from Apple is a *valid* signed version of the firmware that they cannot replicate and, again, once they have that there is no more trusted encryption. To address the “but its only for this particular device” argument, it wouldnt be. After this new branched version of iOS is created it would be trivial for someone (ie - FBI, bad guys, good guys, gov’s) to apply this to any iPhone they wanted.
If you have ten locks that use a common key, then making one key opens all ten doors. The signed software circumventing the security protocols would not be limited to this one phone.