I've been trying to verify my discord account now for almost a month. The error they give all the times is that email provided is already in use. Quite strange for me.

Regarding 2FA, the 12 characters key is what is required to get it working for you. You lose it, you won't be able to login again. Recently, Google Authenticator was deleted from my phone accidentally and I had to recover my 2FA for different websites manually because I had saved the relevant keys for each one.