Input is relative scene exposure, i.e. linear.
To see the S-shape, I usually plot the x axis as log2(x)-log2(0.18). This puts 0.18 at 0 and then units are relative stops. There are many ways though to scale or stretch how the x-axis or y-axis is presented by converting the units and log scaling.
If you need more guidance, I can try to dig up some of my old plots. There’s also a dated, but perhaps relevant post here where the SSTS concept was being demonstrated.
No, though theoretically it should be. This is one of the known inconsistincies that the Output Transform WG is correcting in ACES2. ACES2 will adjust the curve in a smooth and predictable fashion as peak luminance changes, so this is built in to the formula and will be a smooth continuum such that you could enter any luminance within the parameters and get a default mapping that “makes sense” compared to the other settings.