Bell's Theorem So, the high speed cameral has limitations, and when we get to situations with things accelerating at an extremely rapid rate, in an extremely short period of time, as in the case of high energy physics, the high speed camera is inadequate. And, the fact that the assumption of "constant acceleration" is adequate and useful at low rates of acceleration where a small error is insignificant, is not proof that it would be adequate for high rates of acceleration where the small error would be greatly amplified. — Metaphysician Undercover
I didn't give this bit the attention it deserves. You said "the fact that the assumption of "constant acceleration" is adequate and useful at low rates of acceleration" - that's wonderful! If you agree that it's useful and adequate enough at low rates of acceleration, then you've accepted the only thing I really wanted you to. Gravity accelerates things at 9m/s/s, on planet earth, at least for the low rates of acceleration that we measured.
You go on to talk about other instances of acceleration that aren't directly caused by gravity, which I think it's fair to say is beside the point. The conversation is about how gravity accelerates things, not about how your leg muscles accelerate your own body.
You and I both agree, 9.8m/s/s is an adequate and useful idea of how gravity accelerates objects, on earth and for low speeds. And in fact Newtonian physics, which has pretty much the same simplistic vision of gravity as that, was enough to get human beings on the moon! How wonderful.
9.8 m/s/s isn't some perfect magical truth. It's an approximation that works, that we derived by simply looking at the world and taking notes. If you agree that it's useful and accurate in the contexts we generally use it, then you agree with me.