Well, it was silly not to keep the TV box, any electrical store will insist it be returned boxed, so you shot yourself in the foot there to start with. Secondly, they have offered you a repair (despite the dead pixels being under LG's stated tolerance limits) which will presumably replace the panel on your new TV, this is a very common fix for people outside of the 28 day no quibble return limit.
When you grow up a little and look back on this, you will realise how much you were cutting your nose off to spite your face.
I would have accepted the repair if I was not called a liar in store.