Hello again, and thanks for answering so quickly!
As a follow-up, I went back to the main entrance and tried to interact with the front door. Unfortunately, the result seems to be the same as with the stairs: the prompt for hitting the A button appears at the bottom right of the screen, but I can't actually interact with it.
I can observe the coin flattener next to it though (not that it's a particular comfort in this situation).
So, yeah, I guess I'll restart the game altogether.
Thanks again for your quick response and keep up the good work!