For the "Ben" name, I find it has sense. You say that "it dosen't take too much brainpower to know that Ben Kenobi = Obi-Wan Kenobi". Yes, for us, cause "Kenobi" name seems "exotic" to us, but maybe it's a usual name in the Star Wars universe.
If you take "Connor" for example in the real life, how can you guess that "John Connor" and "James Connor" are the same person?
So, "Ben" was just a name for his hiding.

For Luke, I agree with "Yoda Is Your Father", and if we really want a explanation, I think that Leia has a different name, cause the Organa family is famous and has a political importance in the galaxy. Keep her real "Skywalker" name would be too risky...
For Luke, he lives now in a outer rim territory, far from Coruscant and all the political plot of the galaxy. And as for Obi-Wan, "Skywalker" is surely a usual name in the Star Wars galaxy.
After all, there are 3 years between ANH and ESB, and Vader understands that Luke is his son only in ESB, and Luke seems to be famous in the galaxy since he's destroyed the first Death Star.
Again, "Skywalker", in the SW universe, is maybe as usual as "Ford" in our real world.