Could not get 4A ("mode" is very clever), and still don't get 26A. I think I get 21D, but I don't think "write down" alone sells it; maybe something about doing it in parallel? Apart from that, the gimmick is neat (although sometimes rough on crossers), and while a few surfaces seem off, I liked the wordplay throughout. Rather liked 12D, 8A ("narcissistically" is a very economic indicator for this swap) and 15A.