I guess it depends on what you call fanfiction? It's from Opowieści ze świata Wiedźmin, which is a collection of short stories written by authors other than Andrzej Sapkowski in the Witcher universe. The authors have other published works, so I'd argue it's a bit more official than pure fanfiction, but I can't confirm how canon it's considered. Certainly something CD Projekt Red could rip-off for their non-canon story set like 20 years later though.

Also Ciri has been well established to be a headstrong idiot, so I'm not sure why you're trying to examine her decision through rational logic. I'd bet she'd accept the Trial even with a 0% survival rate if she felt it made her a "proper Witcher."