a front door in it which had once been a front door
The intended meaning uses a different definition for each usage of "front door".
The first usage is referring to what the object is, rather than what it can be used for. It is a door, the kind of door that you put in the entrance to a house.
The second usage is referring to what the object can be used for, rather than what it is. You used to be able to enter the house through this door.
To use a similar example with a clearer distinction of the two usages:
This wine bottle is filled with beer.
If it's filled with beer, isn't that bottle then a beer bottle? Well, not in the sense that it's being used here. "Wine bottle" refers to a tall green bottle that usually is used for wine, even though it isn't right now.
I think that the ceiling can't have a front door right now because of the following sentence "in it which had once been a front door"
So you're saying that once the house tilted to a certain degree, the door immediately ceased to be a front door. The logical conclusion would then be that you also disagree that I called the bottle a wine bottle when it contained beer.
More interestingly, what if that bottle had been empty? Would you have allowed me to call it a wine bottle?
The point I'm making is that you're assuming the words used must be currently applicable to the object being described, but that's just not a hard rule that language follows. Sometimes we describe things in a way that they are commonly used, even if that's not the case right now at this very second.