You can always disable GPS at the edges. Or almost always. And, for the rest, yes, they should at least be talking to each other and publishing NOTAMs when possible (and ensuring the FAA has it covered when it's a surprise drill).

OTOH, would it be possible to have other location services such as Galileo, GLONASS, and BeiDou available as backups (or letting they all vote on your position)?

I would hope GPS denial tests also include the alt-GPS systems. Who knows what hardware might have unknown dependencies (one would assume military hardware has all been audited, but assumptions make for poor battle plans.)