Googling led me to this page where the author had the same problem, and they found a solution in Safari Bookmarks Extractor. I can't test that it still works with version changes in both, but it probably does. Once you have your extracted bookmarks, you just use Manage Bookmarks -> File -> Import -> From File.
It certainly looks like it was adblock that was causing the problem. I uninstalled it and I now have a back button. I'm not sure why it was affecting my back button, but it seems to have been the culprit.