So does the Pixel Neural Core chip, in fact, lie directly below the Samsung K4U4E3S4AF-HGCJ mystery RAM in a POP config?

It appears this chip is physically soldered to the PCB - if you had a machine to remove and replace a BGA from it without damage to other components nearby (such as a Martin Expert 10.6 or similary), you should theoretically have the ability to swap it with any other size memory with the same footprint and input/output pins.