I agree that all these things are pointless. There is really no need to explain the Skywalker thing. For one, what is to say Shmi was the only person carrying the Skywalker surname? We don't learn a whole lot of peoples last names throughout the saga, it might not be all that uncommon of a name. Also I don't see why Vader would be trying to scan through the rebels comm channels. He is trying to shoot them down and defend the Deathstar, whatever they are saying to one another would aid him very little in that task. Also a reaction such as "What?!" or "Skywalker!" would be out of character for Vader, IMHO.
I don't know what new dialogue was added in the 2004 DVDs to the Emporer tells Vader he has a son scene, but in the original versions, if I am not mistaken, he tells Vader that the son of Anakin is still alive and Vader acts unsurprised. If we are trying to explain Vader learning of Luke in Episode 4 in order to have him know of him before the Empire Strikes Back, then what is to say that Vader had not suspected the exsistence of his son long before now. Perhaps he was just too stupid to check with his step-brother who was living on his home planet with an adopted son holding the surname Skywalker, or maybe that was just Lucas' lack of ability to tell a believable story.