The argument against the fallback based on these conditions is flawed, it doesn't matter if the fallback is employed or not you're still in trouble.
I was making an observation, not arguing against the fallback.
However I would adjust the language of the ACP to make it clear that in the event of a failure "phpBB will attempt to fallback".
But, another ACP option makes me cry. If this is off by default (as it should be imo) do we really need an admin interface for it? Do we just hide it away as an advanced feature?
I suppose this depends on whether the target audience of this feature is more or less technically inclined.
On the same note, I'm thinking people want buzzword compliance out of this feature (and shipping data to google) more than selecting a cdn. Unless there is actual demand for using a cdn other than google, cutting the options down to "included" and "google" seems like the way to go.
If we do decide to go with just two options, the url should be overridable either via a configuration entry or constant. Somewhat inconveniently, right now editing a constant is probably easier, but when (if?) we implement an about:config-style editor editing configuration there would be easier.
Edit: considering that different cdns apparently munge version numbers in different ways, I'm wondering if the other cdns can be provided via extensions.