Coldhands is not Benjen. Totally different characters, IMO.
I don't know why they have omitted him from the show, I imagine its because they want Benjen to make a heroic appearance. So, yall are likely right on him showing up with some wildlings and some dragon glass and going to town on the army of undead pursuing the litter.
so, that just leaves the big dogs. Who, I would think, would have gotten Benjen by now if it weren't for Benjen being in a group w/ superior numbers.