Well, I can give you an answer but most people don't like it because it doesn't make sense of the Sackett (and other) series Dad was writing. It does however explain what was going on from the writer's point of view.

The paperback business was based on the distribution model of the MAGAZINE business. At a magazine stand when the new editions come in, the old ones go in the trash. Never to be seen again.

No paperback writer from the beginnings of the business in the late 1940s to the end of its heyday in the 1990s really expected his or her titles to remain in print or on bookstore shelves ... especially before the advent of the supersized bookstores like Barnes and Noble. It is worth remembering that very very few writers have stayed in print like Louis L'Amour. Most have the MAJORITY of their titles out of print before the end of their lives.

So, that means Dad didn't take the possibility that fans could buy all his books and read them in order until the very end of his life. At that point it was finally becoming obvious that he was something special when it came to titles remaining in print.

Because most of his career was spent thinking that his books would disappear sooner rather than later he did not write his series with the care that authors do now. He didn't expect them to be carefully compared to one another.

There wasn't much planning to it all until near the end. Some sequences, like The Daybreakers through The Sackett Brand hold up pretty well others raise a few questions. His putting Sackett characters into books like Dark Canyon was probably more marketing (and personal amusement) than anything else. He wanted fans to know that he was going to do more in the future and to keep buying books to find discover additional stories about their favorite characters. He had started in magazines, he knew how to play the game of the "cameo appearance."

So I think the answer is that there was a grand plan but some of it didn't exist in the fictional universe of the stories.