Ian,
You were quite lucky with your heckler, He inadertantly helped you to make a good point. But how curious that he should be in the photography business. Being charitable, perhaps he'd tried to be exhibited and failed.
The difference between a project or series and a group of single images seems to me like the difference between a poem and a dictionary.
The dictionary contains some wonderful words, sure-fire winners in the monthly Fancy Word Competition – palimpsest, inchoate, paradichlorobenzene. The point of a poem is how the words belong together. Nothing fancy about: Wandered, Cloud and Lonely. And yet... The poet will include prepositions which individually seem very dull, until they articulate a sentence. Then they become powerful. (Consider the different implications of "I walked to my home." and "I walked from my home."}
I do understand that this is an imperfect metaphor and it doesn't bear pushing too far. Nor is it an exhortation to make dull pictures. An exhibition may also include some very striking images (20/20 in CC language). The point is that the images reinforce each other.
Footnote: I am using Daffodils because everybody and his dog knows it. It isn't a favourite,