That's the idea I thought to, although there doesn't seem to be a bridge on the third floor.
What happens on the third floor is there is this tile with "paralysis gas" that paralyzes the second party member and behind it is a locked door (which can be opened with a key found after defeating the boss), and behind that is another room with Erick. Unless the player is supposed to use their imagination and ignore the lack of a tileset change.
Which brings me to another question.
though typically translated as "[STRING2] awoke." I'm wondering if I should change it to "[STRING2] opened their eyes." This seems to be the string used when both Sleep and Paralysis are cured.
(paralysis in this game seems to work like petrification in other RPGs, essentially death but without reaching zero HP.)
I'm still quite a ways off, but might as well post the ending text from the ROM.
そして、たびさきで であったひとたちの ことをおもいだしていた。
And then probably followed by some English credits I found afterwards.