Daas Yochid – “The reason herring and whiting are mutor, I believe, is because the amount of worms found are minimal, so there is no miut hamotzui.”
Daas Yochid – “while the heter of the gemora/S.A. is that they are “mino gavli”.
Have you read the details of Rav Belsky’s psak or heard him explain his psak? If you did, it would answer your question concerning “mino gavli”.