I shared the illustration recently.
There's actually a lot of story going on here that's not obvious from that illustration or this sketch (which really does nothing more than lay out the major encounter areas, and note how they're related). The story does, however, drives the whole thing.
In some ways, the way I'm approaching adventure design as-of-late is based on a technique I'm calling the "shreds of fabric" technique. Hah! I just made that up. Look, if Hemingway could use an "iceberg" approach to storytelling, why can't I use a "shreds of fabric approach." Okay, I'll admit, "shreds of fabric" is just my way of saying that little clues are planted here and there, and as more shreds of fabric and placed to together, the clearer the picture becomes of what really happened. I know that's not really a new technique (and those writing professors out there will probably chime in with a more appropriate term for the technique), but it does help me think about how I want the story to be woven as the characters progress through the encounters, regardless of the order of those encounters.
Part of what I'm resolving in this map-sketch (which actually deals with both encounters AND story), is when and how to reveal the story's big plot twist. (I'm avoiding the tendency to want to railroad this kind of reveal. As such, I'll be reworking those "blocked passages" in the middle of the map.)
I'm reminded of the movie The Sixth Sense. Somebody had warned me not to read too much about it before I saw it, because of the big plot twist at the end. Because I knew there was a twist, when I saw the movie for the first time, I started looking for it. Halfway through the movie, I figured it out. Watching the rest of the movie just reinforced my hypothesis. So when the twist was finally revealed at the end, I thought to myself, "Yep. I figured it out." But it didn't necessarily reduce my enjoyment of the story. I'm hoping this adventure is the same. That even if the characters discover the "plot twist" (i.e., get the "whole picture") halfway through the adventure, that it doesn't keep them from enjoying the other little shreds of fabric as they discover them.