Meanwhile, I've made it to page 89 of TStM, expanded a bunch of scenes, found places to add a bunch more, and gotten about to the point where things are actually going to start rolling downhill. Well, I mean, stuff is happening before this, but it's often very quiet stuff, while we set up the dominoes. And on page 89 of the manuscript as it currently stands (it will be a little later in the book once we add the Scary!Danger! and other fun things we need to put in the first part) is where the red domino on the end gets flicked over, and the rest of the novel becomes inevitable.
I think I'm going to plow through to the end, keep marking in the bits I need to add, and go back and do them after. That's probably the best way to handle this.