The big files have kind of a size limit. We encountered errors as soon as the big files were bigger than ~1,5 GB. I know that for example some textures did not show up any more as soon as the file got to big. The problem was solved as soon as we splitted the big file.
Edain is using BFME 2 asset.dat, because there is no asset builder which also adds the specific header the RotWK asset has, which connects it to the original BFME 2 one.
@Matt: You can delete it, but the launcher wont search for updates corretly anymore if you do so. Also language changing via the launcher won't work anymore. You also need to delete old Create a Heroes, you can simply restore the "My Rise of the Witchking files" folder via the launcher if you click on "Fix a non starting game", but be aware that it deletes the whole folder, so save your maps and replays before.
@Naz: It should not interfere, but I did not test it yet. You need to disable Edain for launching SEE for sure.