Odd thing happened.
Facebook must have changed something on their end because Disa stopped working. As I needed some sort of functional client for this service installed, I reinstalled the real Facebook Messenger.
All the "fluff" was missing. The "Days" thing was not there, the centre button at the bottom to use Snapchat-like camera filters was not there, and swiping down refreshed the list as it should instead of opening the camera.
Next day, Play Store downloaded an update for Messenger, and now it has all the garbage in it again that I don't like. So it would appear that when you initially install FB messenger it is absent of these features and soon after, it will update and gain them.
I have re-tested this, uninstalling and reinstalling it, and the same phenomenon happened again (well, it hasn't updated again yet to gain the extra features, but I'm sure it will soon).
Has anyone else observed this? Maybe there's a way to trick it into only using the lower-feature form of each latest version.
Update: I had a friend test and he experienced the same thing. Then we both observed that the Messenger app internally enabled the extra features without any update downloading through Play Store.
So, the app has these components within it and they can be turned on/off based on certain criteria. I wonder if it's as simple as an on/off tag in a configuration file somewhere.