As Stoffel said, it's out of Microsoft's hands.
No, it is not. I'd argue, 99,99% of the code that they include in this and pretty much every other update does not touch anything that depends on a carrier's own implementations, which are in most of the cases non-existing, except for some silly carrier-specific phones, which would only concern the carriers that manufactured / ordered / whatever those phones.
The only thing that could interfere with a carrier's everyday compatibility would be a bad firmware driver for the antennas / radios / whatever which are used to connect to that carrier's network. The rest of the code, especially the user-facing code, can easily be deployed to every WP8 device, just like one can easily deploy Windows updates to every Windows device. The carrier-specific firmware could afterwards, after being proven to be 100% compatible, something which I doubt carriers even bother to do as they have proved time and time again that they are way too lazy to press a stupidly silly button to allow Microsoft to distribute a particular update to their users (hello WP7 and Android users), be deployed as a package separated from the main package, in this case the 8.1 version of the OS.
There is no logical reason for the non-carrier-specific firmware / antenna and radios drivers to not be distributed to all users on the same day, worldwide. But then again, there is no logical reason for Microsoft to be doing hundreds of stupid things that they're doing at this very moment, but still continue to do them... (in particular, everything Xbox related, everything transactions / Store related, and other account regional locks related crap) but that is a story for another day.