As to reading order, Lewis may have been tailoring his answer to his questioner. (We're told Jesus did a lot of that too!) The earliest reference I've seen, was in a letter answering a child's fan letter. The authoritative one was in answer to a US publisher who wanted to put numbers on the spines (common practice with series). So there may have been an element of "Well, if you have to ASK somebody, you're too young for Publication Order."