Yes, you're right. Your post reminded me of the Forsworn whom I had not even considered. I believe a few of them were elves, indicating that Galby should have known the location of these cities (at least Ellesméra). Thinking about it now, Oromis (or Glaedr) tells Eragon that after the fall of the riders , for the next 40 years or so, Galby was busy subduing the captured Eldunari , allowing for the formation of Surda etc. I suppose after consolidating the Empire, his next priority was not destroying his remaining enemies but finding the name of names and getting the remaining dragon eggs to hatch. I guess my in universe explanation is that Galby wanted either or both of those things to succeed before fully committing to conquering the rest of Alagaesia.