Is there a 3D Alef-Bet? In which case Samekh could be something like a sphere, etc.
Or for that matter something 4D like kinetic sculptures, which would explain how the aseret ha-dibrot were legible from every direction . . .
Could this be related to the form of the actual sound waves in the air when the letters are pronounced? In which case people would literally be seeing the physical forms of the sounds, explaining the synesthesia as well?
It's essentially impossible to prove that nobody has ever proposed the existence such a 3-dimensional alphabet, but I have never heard of such a thing and I think it's safe to say that no major religious scholars have ever proposed that such a thing exists.