One ending "In the Clutches of Chaos" gives the investigators 4 options, one of them being Anette teaching you "spells of old", if one investigator has the "sorcerer" trait. We did choose this because Marie was part of the group, but it wasn't referenced in the last scenario. Does anyone have an idea whether this is relevant? Did we miss something?

I think it's just in there thematically, and to hint at the cyclical nature of existence (ie the theme of the entire cycle) rather than to have a specific effect. Matt mentioned in his Circle Undone wrap up that he wanted to include some trait interaction, but more from a roleplay point of view than necessarily to have a mechanical impact. (You can hear it here: https://drawntotheflamepodcast.blogspot.com/2019/09/episode-138-in-azathoths-domain.html /shameless plug)

I see. I was just wondering because the 2 options that everyone could choose, had an effect on the ending, while this one wasn't relevant at will. I expected such a special choice to have more impact than a generic one.

I mean, you could always homebrew something. Like a free XP-upgrade for a spell, or the option to switch out some L0 cards for free.