He woke up, understanding at once that the whole purpose of the dream was to remind him, with diabolical roundaboutness, of Erlys Mills.
**********
"diabolical roundaboutness"
Pynchon describes the way the dream works to get its point across in much the same way almost anybody would describe a book like Gravity's Rainbow. My first thought was, "That's an apt description of House of Leaves but then I remembered I'm reading Pynchon and Pynchon wrote that line and Pynchon most assuredly was saying in this moment to his readers, "You want to know what the point of my book is? Well, we'll get there! At least I've limited my books to several hundred to a thousand pages which all come out as one complete story. You want real diabolical roundaboutedness? Read Tristram Shandy!" Okay, Pynchon probably was most assuredly not saying all that.
Anyway, aside from the strict definition of diabolical which suggests terrible evil as opposed to the way I read Pynchon's use here as a sort of "evil genius," I'd say "diabolical roundaboutness" fits the way Pynchon gets to his points using great intelligence and whimsy. It's also probably why so many people find his books daunting! It's a good description of Pynchon's writing style.
"Erlys Mills"
Obviously the dream was going to be about Erlys and Merle's heartbreak since the paragraph began with how the dream took place not long after Erlys left him for Zombino.