You may try to remove manually everything again, and to create the mysql user by yourself:

    sudo bash
    niutil -create / /groups/mysql
    niutil -createprop / /groups/mysql gid 401
    niutil -create / /users/mysql
    niutil -createprop / /users/mysql gid 401
    niutil -createprop / /users/mysql uid 401
    exit

Then try to perform the installation again.