PAA: Your example was not of something Patur Aval Assur. By Mapis Mursa, it’s either Chayav Chatas or Patur UMuttar. If it’s L’hotzi Leicha then it’s Ein Tzricha L’gufa and Chazal weren’t Gozer in this case. If it’s La’asos Pesach, it’s Chayav because now it’s Tzricha L’gufa.
Chacham: I don’t know if you’ve seen or mentioned it at all in this thread, but Tosfos Shabbos 75a seems pretty clear that the Chilazon is a fish. Any thoughts?