Yeah, there's even more to that event than you listed.
Even if you agree to go on the adventure with Briony, your plans will be ruined if you've done any of the things to trigger the coup d'etat by the other nobles. That ruined at least one or two of my attempts to complete the Briony event. On the plus side, if you've convinced Briony to go with you this far, you can use her as a hostage to get the rebelling nobles to back off, since her own parents are among those attempting the coup. Needless to say, she won't like you if you do that, so no yuri option for you.