Welcome, thestumbler!
Yes, the bottom two lines should remain in place when the screen updates. I believe Ian has some fancy code that only redraws changed portions of the screen, too.
However, other users attempting to use screen have run into problems, and the general consensus was that other programs (e.g., tio) may provide better experience.
The secondary plane is not yet being used, although it’s being considered for either “menu” style popups, or for when changing into mode-specific display of data. I don’t have the thread for that discussion at hand.