Thank you so much! The running dialogue during gameplay took quite a few tries to get to work properly (something I hope to elaborate on in a Dev Log eventually), but basically it makes use of "Unlock Script" and "Draw Text" to write the text directly to the background, that I then reset the background to its initial appearance.
The "sprites" for the cutscenes are in fact background tiles as you guessed. The only things that are sprites are the eyes and mouths. (And Rab-bit on the occasion he floats in.) The fade to black is accomplished by changing the palettes, which also hides me swapping in the tiles of different characters.
I feel like too much of the project is in flux to share the source, but if you have any other questions, feel free to write!