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Source: https://writers.stackexchange.com/a/33267
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

A few tricks come to mind. One that your description strongly suggests is similar to Agatha Christie's "And then there were none", which might be exactly the kind of work you are looking for. It matches your description almost exactly.

In that puzzler, she had the same issue - couldn't disclose during the story (would not work if she did), yet too complex to sum up quickly.

What she did was have the perpetrator leave a diary or letter for others to find, which as well as explaining without breaking the story, also _added_ to the book by showing you the whole thing back to its origins, from the perpetrators viewpoint. (The perp left a complete description close to a mini autobiography, in a bottle thrown out at sea, an left it to chance if it would be found. Similar might be a letter left with lawyers to be sent in 30 years time, or buried with something - or in the digital age, an encrypted message left on social media/hard drive/email, to be cracked when technology allows, in 30-50 years.)

It worked very well, so it might work for you, too. Often such people want the world (or someone) to know, some day..... so it can probably fit in well in many kinds of plot.

You ask if anyone knows a story where a massive explanation follows the final scene, and does it work/is the book still gripping? This book is reckoned one of her masterpieces, and the answer is an undoubted "yes". Not an overly-long book, but superbly mysterious, and an "impossible" whodunnit - go get it on Amazon and enjoy, as well as seeing how it's done! Hopefully a hell of a read as well as an answer :)

Original score: 7