Phil535, My interpretation of the first instruction is that it consists of four words plus 2 letters. That's the bit I don't understand. Why is it necessary, especially as the preamble tells us to use all cells in the conversion? If the first instruction is carried out won't there be two cells short?
The last instruction is indeed ambiguous. Meursault's guess is probably right, but the last word of the thematic item is enough on its own as well.
Last year the setter's puzzle on Dante's Inferno had various ambiguities, so the editors had to allow alternative solutions. Is the same going to happen here? And how brief is 'brief'?