Oh, of course the innnocence was gone. The hobbits especially were forever changed, and the Shire was too. But that is something that too has been talked about many times in the B-D's before. And the "particular light and joy" passing from Middle Earth that you talk about could be the elves (or are you talking about just Frodo?). Because the War of the Ring brought the passing of the elves and that was definitely the taking of a "light and joy" that would never be seen on the shores of Middle Earth again.
Loss in victory is something that Tolkien has threaded through many of his works. I guess you could compare it to someones experience in war. You may have won the battle overall, but you could have lost your friends, and you have most definitely lost your innocence.
But I can't really answer the last part though, because that is just a thing where I can't seem to find a good example for. I think it maybe exsists, a conflict where everything and everyone comes out good, but maybe just in a perfect world
