However, even with the content of the second column removed and using the
@line-height="0" I still get a small blank space *after* the graphic above the
bottom border line of the cell.

I guess this is due to the +1.5pt in the row height. You should try
without this addition, and only if you get an error indicating an
area overflow you might add a small amount (0.1pt or less).
Maybe there is a bug related to the 5pt padding in the cell above,
so omitting this might also be worth a try.