Sorry again about the delay, I was on a vacation trip.
I have not said that the MTF computed using the slanted edge method isn't useful. However, I have said that the MTF calculated with this method isn't scientifically accurate if you want absolute accuracy. The problem with the averaging is that it tends to lose information of the spot itself, while the average along two orthogonal directions is computed with sufficient sampling, pretty much nothing is said about what happens between the orthogonal directions.
For this reason, I don't believe it would be possible to reconstruct an accurate PSF with the slanted edge method and thus the measured MTF must be slightly invalid as well. You can think of this from the dimensional reduction point of view; it is generally not possible to recreate a 3D function from two 2D functions. Higher order aberrations do give rise for all sorts of interesting spot shapes and orientations with element decentering.
We are getting here to more philosophical questions. In science, we use modeling. We make some a priori assumptions, ignore this and that, and then build a model which we analyze. That model is never perfect, and it can't be. We must be aware of its limitations. But saying - you can never have a perfect model, so why bother with science at all - is not the right thing.
Long time ago, Riemann suggested to model anisotropic phenomena by a quadratic form. The level curves (in 2D) of that form are ellipses. 2 measurements then are enough. In some sense, this is equivalent to taking a truncated Taylor expansion of a more complicated function.
Going back to photography - when the PSF is well concentrated, approximation by a quadratic form is OK. When it is not, you can see it, no need of sophisticated methods (like my 35L at f/1.4 in the corners, wide open). Those are some of the limitations in this case.
So when I say - you can get the the PSF from the MTF, I always assume some reasonable model in place. Also, I do not mean that you must use the DXO test, necessarily. If you are curious enough, you can rotate your target, and get get more directions.
It is relatively easy to think that there isn't differences between the behavior of rays when shifting from a wavelength range to another. I hear this argument quite often, and this may sound like blasphemy for some, but I disagree with that.
In the good old times, DXO published MTF charts on each of the (RAW) RGB channels. They were different enough, indeed. Actually, sometimes, too much. They reported some kind of average, weighed heavily towards greens, if I remember well, because people want simple answers. If you dig deeper in that, the spectral decomposition of the light in the test would play a role, too, etc. In principle, the camera projects an infinitely dimensional color space to a 3D one, so full spectral information is lost anyway.