Interchanges can be made when defending, it's just a daft thing to do as you are a man down while the interchange takes place.
An interchange is not normally allowed on the pitch until the person they are replacing has left the field completely.
Also as far as I'm aware changes can't be made unless your the team in possession of the ball (or at least that's what I'm told in Champ 1). So the 14th man would have been on for a while if that's the case.
1. Salford put the player on without going via the official.
2. The interchange official has allowed the interchange without making sure a player came off
Either way, no replay. Technically, it's no worse an offence than any other in the law book that goes unseen by the officials.
I have done this job and have had players throw the interchange card on the floor and try to run on without waiting for the player to come off. At that stage (if the other player stayed on) you should stop the game but that's easier said than done as the ball may be in play and some distance away. In this example it would be a penalty to the non-offending side where the 14th player entered the field, 10m in (I think). Never seen this type of penalty given.