Let's not beat this poor dead horse any more. I searched for columns-flex in https://github.com/enketo/enketo-core where I'm pretty sure it would be defined and it's not there which suggests Enketo has not gotten this change yet.
@martijnr is the one who suggested columns and columns-flex here so let's go with how he described them there which is option 1 above. I have edited my post here to try to minimize confusion (hah).
< snip obsolete grid >