Roland's mother is Odon's sister.
Roland talks about his family a lot, particularly his mother, implying she is important by the Law of Conservation of Detail. He also says he is from Norway, and Odon recalls his sister ran off to a foreign country. At one point, Roland also echoes the "collection of mistakes" line, attributing it to his mother, which is pretty suspicious when it seems to be signature phrase of the castle.This would also explain why Roland is tied to the heir conclusion, and could also explain the purpose of his urn. Perhaps it's a canopic jar for his mother, whose last wish was to be returned to her birthplace? Heck, maybe her name was even "Tia", which would make Roland saying the urn is "Tia's" Metaphorically True. This would also imply that he knew a lot more than he let on about the heir subplot and was stringing Flore along for most of it...which is consistent with his conveniently discovering the smoking gun and giving Flore leading questions towards figuring out the mystery.
The castle is never claimed in the ending, though, which implies he either doesn't know he's the true heir or he doesn't care.
The first FOIL was the caterpillar charm.
Each option corresponds to one of Squeaker's stories, and the caterpillar story feels like the oldest and most significant. The charm could probably be considered representative of the concept of materialism itself, making it a good candidate for the seed of FOIL whether literally tied to the story or not.