marshallbI just spent 3 hours trying to figure it out, and your diagram is right. Unfortunately it seems I don't have enough 1x row 3 keys, I seem to have gotten extra row 3 keys instead! So I have only 6 of the 1x 3s, and 4 extra of the 1x 2s.
edit: and also a keycap puller is SO needed for working on this thing, it would be pretty dodgy trying to brute force them out with fingers.