You can't give corrin the ability to use a dragonstone, only the noble class. You would have to recreate a class with the ability to use the dragonstone, similar to the witch class deathchaos made. Also, you don't need to backup your ExtraData at all. Just delete the DLC when you're done using it and just install your other DLC. You can rebuild them into 1 big dlc though.Could you make a custom dlc version of Corrin that can always use a dragonstone? Or really any other custom characters who get additional always able to use certain weapon flags?
Also, if anyone wants to use this, the cipher characters, or any other DLC characters in another region, back up your ExData from the region that has them usable, as well as the ExData from the region that does not. Then copy the files over to the ExData for the destination region and restore the backed up ExData. I was able to use the Cipher characters and DeathChaos25's custom DLC characters on a EUR version by doing this. After you've done that, you edit your save to include the characters.