Rebbe,
Why would anyone have a hava amina that if individuals, whether one, a few or many, ate cheilev or any chiyuv karet b'shogeg would not be chayav a chatas if the Sanhedrin had not ruled on it or said laasot?
B'kavod,
Sam Kosofsky
Shalom Reb Kosofsky,
It's great to hear from you! Yes, as I believe you are assuming in your very perceptive question, the individuals who violated are not going to be Chayav a Chatas if the Sanhedrin did not issue a ruling. Granted, one may have mistakenly thought that if the individuals overheard the Chachamim saying that it was Mutar, even if there was no Hora'ah, then the individuals would be Patur, since in a sense the individuals are "relying" on Beis Din who "declared" that it was Mutar. But again, as you correctly indicated, l'Halachah there is no Petur for the individuals unless there was an official Hora'ah l'Ma'aseh.
May you continue to attain greatness in Torah and Yir'as Shamayim!
Warmly,
Yishai Rasowsky