China (or Russia or Germany or whoever) could force Apple to backdoor phones used by CIA informants in that country.
It's a ticking bomb, man.
This carries with it the assumption that the digital signing and verification mechanisms are infallible and impervious to attack. That is an unwise assumption. Even if a software system appears to be perfectly secure at a given time, it is reasonable to assume that at some point a vulnerability will be discovered.
Not necessarily. Someone could get their hands on the signing keys or find a vulnerability in the signature verification without having the knowledge or resources to create something worth signing. Or figure out a way to bypass the check by changing something that isn't covered by the signature, or use something like rowhammer or hardware hacking to flip the bit from saying the check failed to saying the check passed, etc.
And if you can get around the digital signage, you don't need the compromised OS.
Conway's technical interpretation of the Apple deliverables is right. There's a legal precedent which could cause reuse (and is rightly matter for debate/utter refusal of the FBI position), but if you just debate the technical merits Apple has been very misleading about the consequences.