Those two letters are correct, Lumen. And yes, it's a two-part word. Perhaps you've miscounted when doing the alphanumeric calculations? Easily done. And don't forget both the answer and entry go from right to left: I made that mistake.
Planks, I get the impression nobody has the actual connection between the thematic items and the writer, although as Gitto said earlier, the items are confirmed by the hint from the extra letters.