The problem, as I understand, is that Apple doesn't have that ability. Apple doesn't want to create that ability because the potential for abuse is there.
Exactly. They can't do anything on a 'per phone' level without a massive software rollout, IIRC.
The FBI isn't asking for a key per se, but disabling one security feature. To use an analog, they're asking Apple to remove the glass plate relocker in the safe door. Of course, with software, that's possible.