The bottom row is quite specific, and doesn't define the full set of six (perhaps this is what quixote has been getting at?) but the preamble is clear that the three pairs are thematic in a different way to the set of four. However, I think the setter has gone to a lot of trouble with this slightly whimsical selection of things that can precede the theme word. Each is a combination of two of a kind that differ in a key respect (this is most obvious in the central pairing, where they're actually the same thing in everything but...). The other two pairs are similar and, for all three pairs, the major partner is the major entry. That's quite a coincidence if it's accidental.
Not that any of this matters, because the set of six is a given - we don't have to do anything with it.
I don't submit my solutions and I wouldn't want to push my version of the endgame - where 'artwork' is being asked for artistic licence should be tolerated (even if it's a Cubist interpretation}.