Thanks! Although, maybe I'm just totally clueless about how these file structures are set up, but is there some reason it wouldn't work to just copy the relevant AID bits from the amiibo versions of the characters?1. Nope, the Cipher Characters are silent, adding their voices would require editing their AID and AIDs are a pain to deal with, and if you don't know what you're doing, or even if you think you know what you're doing, one wrong thing in the AID and the game will crash.
2. Correct.
3. It depends, if you're using the extdata version it would be rather difficult because of how filesize and compression work, so editing the files might cause the game to crash every time it attempts to load them from extdata meaning you'd never get past the loading screen and into the title screen.
If you have CFW then it'd be as easy as going to romfs/global/GameData/Person/ and locating the 3 files for the Ciphers, decompressing them, recompressing them and then just rebuilding the DLC with the changes.
I have no idea how avatar units work, I know they have a character block like any normal unit, and then the boon/bane adds/subtracts to those growths.
Also, I'm still curious as to whether the DLC will still work when the game is booted normally rather than through homebrew.