Names of V1 characters are permanently locked to the account on which they were created even if the character bearing that name is deleted, so only characters created on that account can be given that name. SE did this to ensure V1 players didn't 'lose' the names they used in V1 when V2 launched.

So, even if you manage to delete the character, which as you've found out can only be done in-game, you'd not be able to achieve what you're trying to.