For one thing you keep insisting on 'motion' through the spatial and temporal dimensions.
If you take relativity seriously, there is no space, and there is no time; only space-time, as the two cannot be separated.
The argument is that you can neither move through time, nor move through space, but you are at ALL locations you occupy in space-time.
In a simplified case, as you used, where we consider only two dimensions, displacement horizontally and duration vertically, you exist as a line through space-time. Vertical if you are standing still, with a slope if you undergo a displacement.
And if your axis are marked in seconds and kilometers, the slope of this line can never be less than 1/300000. IOW you CANNOT move only in space.
In this example, 'now' is a point on the line, and differs for every observer; that point, 'now' is what each of us perceives to be moving, through space-time.
I can't really explain the 4dimensional equivalent, but you exist as a volume, 'extruded' through a fourth orthogonal time dimension, and your 'now' is a hypersurface or 'foliation' of the space-time manifold, and again, none are common to differing observers.
You are trying to understand 'motion' through time, and inventing all these complexities, much like the 'add-ons' to the Ptolemaic System in its later years before Copernicus replaced it, to try and explain things which are not even required.
Take Occam's razor and cut away everything that isn't needed.