It sounds like writers were (and probably still are) under significant pressure to stay in their "lane". Fortunately for us, your dad was able to sometimes blur the lines. At least, I think so.

Which gives me another question. Most of your dad's protagonists were along the heroic lines. But would you consider the Sackett twins to be examples of antiheros? And, if so, are there others of your dad's novels that you would consider to have antiheros as the protagonist?