"the expectation is that developers handle split-screen and freeform exactly the same" -- users may disagree with you.
"If you have work that is a logical continuation of your task, you should continue to have that as part of your task (and hence, window)" -- again, users may disagree with you. Freeform multi-window does not have to exactly match the WIMP model that Windows and macOS users are used to, but the closer it gets, the fewer support issues developers will face.
"Do you have specific design configurations (aspect ratios, etc) that you don't see covered through testing split-screen multi-window?" -- everything needs to be re-tested on freeform multi-window. We won't know if there are design configurations that are an issue until we actually see how things work. That's why we test things. This isn't a question of known unknowns; it's a question of unknown unknowns.
Outside of a few sentences in documentation and random Medium posts, we have zero
official information about freeform multi-window. The fact that we lack information is typical. And, we'll hack solutions around that lack of information as best we can, as we do with everything else. And I'm not surprised by the lack of freeform multi-window mode on Google devices. However, one would think that it would be in Google's interest for there to be some official testing solution for freeform multi-window before major manufacturers start shipping freeform multi-window devices.