I've read this elsewhere... there's an expression that someone is so useless they 'couldn't run a 23 D stall', so that would fit the 'most incompetent will fail to shift'.
No, your crossers are correct. You need to put an abbreviation for 'king' inside IT reversed ('in it when erected') and follow that with a four-letter word meaning 'shortly'. The whole thing (when preceded by 'Le Grand') is a French palace.