Hi Cockle
I believe the whole puzzle is a representation of the Rosetta Stone.
The thematic diagonal is Demotic.
The Rosetta Stone has hieroglyphs at the top, demotic in the middle and Ancient Greek at the bottom.
I think the hieroglyphs in down answers are to be entered in the top you then fill out with appropriate other hieroglyph and another animal to spell out the name of Champollion (who cracked hieroglyphs). The pronunciation of the hieroglyphs we have from downs seem to sound out his name.
I then assume that the initial letters of the rectangle are converted to their Anciemt Greek equivalents.
If you then follow the instructions with the highlighting the result seems to represent the shape of the stone itself.
https://en.m.wikipedia.org/wiki/Rosetta_Stone
What do others think?