We even use the Lorentz transform, developed for post-Michelson aether theories, precisely because you cannot tell whether you’re in a varying aether or a varying geometry.

We define distance and time relative to c, so we cannot measure it in the usual sense.

We know we've got it pretty well close to accurate, for the bulk of the observable universe.

If c changes, then chemistry changes, and stuff like hydrogen absorption lines shift. This is how scientists have looked for changes in c in the early universe.