I feel for you. I too have lost several files, but not as large as yours. I seem to remember Pages on my iPad telling me something incomprehensible about needing to upgrade on my Mac, and it was just after upgrading, I think, that my files vanished...right before my eyes...with the "deleted at another location" message.
Like you, the /Library/ suggestion has come up fruitless.
What with iCloud eating my files and the NSA snooping with the connivance of Google, Apple, Microsoft and all the rest, my faith in ALL forms of Cloud computing is now zero. I will never trust my data with any of these clowns, so it's back to USB sticks and external hard drives for me as back ups.
What is even more remarkable is that this thread is now more than two years old, and Apple still haven't fixed the problem. AND their iCloud still labels Pages as Beta - so they know it doesn't work right, and they're quite happy to let their customers have hours of hard work destroyed in an instant.
They used to say about Apple products, 'it just works'. Not any more.