This is noraaki's yuniverse so let's roll with that
I only got the general sense of the comment so it might just talk of this specific Marisa
It's a metaphore to say that stray koyukkuri only ever find one treasure during their whole life
#3 is pretty much my takeaway. Literally, the only easiness a wild yu will find is in the trivial rejects of human society. A piece of Easter candy never found, a penny that fell, the box that made a home.