You have a talent for picking out the weird events, Deathlike2. Turns out the sick room in Toroia castle is another map that loads characters from the underground/moon set rather than the overworld set. I must have missed it in my first pass over the data. I traced it back to another event flag that I didn't know about. It turns out that the in-bed Edward actually has 5 different dialogs, depending on the event flags set.
(The tracing goes like this: I used the character set in the location properties to look up Edward's character number, then looked up his speech data using that number, then decoded the speech data to find out in which situations he says different things. One of them was the event flag that I didn't know about, so I searched my event offsets for that event flag to find out which event sets it, then searched the speech data once again to find out which character loaded that event.

)
And the final answer is: Deathlike2's guess is right. It's loaded when you speak to a Sylph in the Sylph's House. Second mystery solved. Another

to me. And a

to Deathlike2, too, since he was pretty much right on.