They've put themselves in a weird legal situation because they've made it so that they are the only ones who can actually write and sign the firmware the FBI is demanding. A judge would laugh them out of the courtroom if the FBI was technically capable of writing the firmware and demanded Apple's help because it was too hard.
This is an example of a non-free software feature. Why are the keys baked in and can't be disabled. And "write your own firmware" doesn't solve this problem -- they could just pay a developer to do it $X an hour. A better security model should've been used -- where updates have to be confirmed (read: signed) by the user before they are applied.
No, not based on the interpretation of the all writs act that the FBI is attempting to use. As far as the FBI is concerned, they could force my Grandma to write a backdoor if they deemed her the best person to do so. Given that she can't answer the phone most days it'd be a lon wait, but I wouldn't put t past them.
--edit to correct auto-correct
The only reasonable way for law enforcement to deal with even a single one of those factors is to request help from device vendor.
Closed-source may be a pet-peeve of yours, but it has nothing to do with any of this.