Hi Dryden,
We are in agreement regarding the shape and position of the rhombus. You can if you want prove mathematically that the sloping sides have the same length as the horizontal sides (the equivalent of five cells).
If you compare the positions of the top left corner (in the first cell of 1ac, containing the letter B) and the bottom left corner (in the fourth cell of 20ac, containing the letter E) of the rhombus, the bottom left corner is three cells to the right and four cells below the top left corner. Making a triangle of the top left and bottom left corners and the centre of the cell perpendicularly above the bottom left corner (this is the fourth letter of 1ac, containing the letter F), we have a right-angled triangle with sides equivalent to three and four cells, so by Pythagoras' Theorem, the other side must have a length equivalent to five cells, as is the case for the two horizontal sides.
I think that Meursault was trying to draw the sloping sides of the rhombus as passing through the centres of ALL the cells that these sides traversed. It's possible to show mathematically that no rhombus can be constructed that satisfies this condition together with the other conditions of the preamble, but as it's rather late I won't set the proof out.