Another option the FBI has is to issue a search warrant for Apple's code signing key. Then they could flash the device themselves. That's what they did to Lavabit and it has a lot more legal standing than this All Writs Act thing.
I do not understand why no one is talking about this possibility. This is a much more terrifying option, the precedent already exists (lavabit), and it would enable a much broader application of different versions of FBiOS.
If the FBI were to do this, would it be possible for Apple to counter it in the future by splitting the key across multiple entities, some in non-US jurisdictions, in such a manner that all of them would have to agree to sign anything?
You just described David Chaum's crypto plan. Also widely derided as having a backdoor. So it's really just moving the goal posts.
It's kind of an interesting idea. Doesn't seem like it would work for RSA cryptography, though -- in that model, your private key is a pair of prime numbers, and your public key is their product. It's hard to produce both factors from the product, but it's pretty easy to produce one factor if you know the product and the other factor.