Hmm. Back to a very long list of entities then. The lower light in col1 is defined in a way which suggests that a 6-letter word (not vegetable or mineral) might go in there upwards. And a real word emerges. Too overthought, obviously.
The lower light in column 1 comes into the same category as the lower lights in columns 9 and 13. In some ways it is in its nature/content the antithesis of that in column 9.
I am about to throw in the towel with this puzzle...
I have the theme, , mostly. A hint for 21d might help if anyone can assist please. I have *L*D*M. but they could be all incorrect.