Whether you declare your past or not is your choice. However, if an employer ask you, it's usually best to tell the truth. I suggest you talk to your present/future employer before seeking a new contract. Also, keep a copy of any communication regarding this so you have a copy of each message/letter you've sent and received, these could prove very useful to you just in case immigration or the local police ask. You were honest and told the truth from the beginning when asked.

As far as I know, pardons are only granted (in most cases) to anyone who has been convicted of an offence. No applicable to you. You're clean.

Enjoy korea

Jack