As I understand, when referring to a single concept, one would use "ham and cheese is", but "fruit and nuts are".
Now, can one have a single firework, or is/are fireworks simultaneously singular and plural?
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Fireworks is a collective plural noun which normally takes plural agreement.
The fireworks are going to be beautiful this year.
However, this noun is defective in the singular, as the singular "firework" is very rare.
Your other two examples confuse the issue. Two singular nouns connected with and normally take plural agreement:
Ham and cheese make great sandwich ingredients.
Ketchup and mustard taste good, too.
The only time that "ham and cheese" could be used with singular agreement if it's being taken as the name for a particular kind of sandwich.
The ham and cheese is great in this deli.
This doesn't really have anything to do with fireworks, which is an ordinary plural noun for the purposes of morphology and syntax.
Fireworks is a plural noun with an 's' on the end, so it is plural fireworks are.
The same applies to your fruit and nuts are example. The 'nuts' is plural. In your ham and cheese is example, both nouns are singular so it could go either way.
Ngram because I enjoy them.
There seems to be a very specific case where you frequently use the singular. The context is normaly admiration: "but second half was a real firework with four goals" "this event was a real firework for us" "The last wedding of the season was a real firework of joy and dance !" "It was a real firework of new and old songs" Dora was a real firework on stage, making us laugh out loud..." ...and more. This use seems to be frequent in literature, and I see many examples in sports commentary.