I don't think he has decided the endings, in fact I've seen very little that would convince me that he has more than a vague outline as to where the story should go.
That comes from his style of writing. DPC writes the story backwards, I didn't know that but I was reading the AL thread and I found a message from the Dr. admitting that he wrote the story backwards with AL and now everything makes more sense to me.
When you write a story from the present to the future (but you haven't decided the ending, just the present setting) things are very cohesive because the things your characters are doing shape the future. You don't know where and how the road will end, you are just building the road as you write it, but the road is destination unknown. This is the classic way of writing episodic media like sagas and tv shows. When a tv show starts, they don't have a clue in season 1 of what's gonna happen in season 5 (it may not get there if it's cancelled).
Now, when you write a story backwards you know how it ends and what are you trying to accomplish but you have to think of the story that is going to take you there, so some things can feel like a vague outline because the effort is not on the actual things happening, is in the direction you want them to take. That's why some characters have changed, or some things are poor plot devices (like Maya's loan) or screw ups, they are things you need to do to make the plot advance in the direction you need to get to your ending (For example Star Wars ep1-3 was written this way) and sometimes can feel forced, out of character, etc.
So, he may not know every detail about the endings (still has to write them) but I'm sure he has a good idea of what he wants.
On the positive side, this style of writing gives you a dynamic story because you feel things moving. And because he has thought of future events he is able to drop here and there all this little details and clues that we love, this thread would be 4k pages shorter without them
