Free your mind of doubt and danger...
Byron's puzzle is undoubtably about the relationship between Man and the Fair Folk. He says it best on Page 7:
you will learn of your role in the Fair People’s story,and the significance of the quest for their treasure in therelationship between Man and the Fair People.
We should all be able to agree about that. Though we may vary on how we interpret what "significance" means.
One place that we can look, to find what Byron said Man and the Fair Folk, are the two treasure poems that bookend the paintings and verses.
Let's talk about repetition. He uses 3 different words that represent the same concept. Pact, Accord, and Wed all represent "a formal agreement between two parties". Feel free to fact check that on Oxford or Merriam-Webster.
On the surface, he is giving us a simple instruction, you will have to match a painting with a verse. At one level of abstraction higher, he is saying that the matching of painting to verse is "a formal agreement between two parties". In the context of Byron's puzzle, those two parties are: Man and the Fair Folk.
He goes on in "A Tale Simply Told" to describe features of Man and features of the Fair Folk. We are told that Man "possess names" and "finds his strength in words". We know there are a lot of names of Men found in the Verses (e.g. Twain, Seloy, Man of oz, etc) and we have also found the "words" of Men, i.e. quotes, in the verses (e.g. Stevenson, Mellville, Sarmiento, etc). Man is associated with the verses.
We can plainly see the Fair Folk in the paintings. Byron describes many of them at length in "The Passage To the New World".
The next instruction we get, is in "The Litany of the Jewels". Byron says:
of all the desires of the Fair People, there is one thing for which Man shares the same passion: Jewels.
The jewels are what bring Man and the Fair Folk together.
Here we have the first part of Byron's puzzle, or how to pair the verse to the painting:
Find Man in the verses.
Find the Fair Folk in the Painting.
Use the Jewel (i.e. birthstone, birth month, birth flower) to bring the two together.
'Cause tonight is the night when two become one...
 Pact and accord both mean roughly the same thing, a formal reaching of agreement between two parties. Wed is no different, as it is a formal agreement between two people.