[Phlarp]

I suspect there are a myriad of technical means to pop the phone, including the method you outline, but I doubt the FBI has the talent or connections to get it done in way that would be presentable to court. (I.E. Nobody in house is capable and any contractors they would hire to do it don't want their methods made public in an open trial)

One assumes the NSA would make short work of this phone's lock if it had been recovered from the OBL compound, but using them in this instance also brings in some dicey legal issues (they "can't" operate domestically) and the NSA is even less willing to give up it's own tactics than a security consulting firm would be.

FBI seems to have chosen to go the "lawyer up and look for a court order and some wet blanket executives willing to hand things over" route-- Tim Cook is, thankfully, well principled enough to tell them to pound sand.

Surely, at some point in time, the FBI will figure out how to recruit technical staff that is capable of doing more than just extorting bitcoins from cyber drug kingpins.