Is Eating Niyaz, Or Earning Of A Sin Equates To Sin Itself?
Disobeying God knowingly for a major sin is considered kufr. Shirk is associating partners with God. Eating a food associated with some other gods, without the belief in other gods, cannot be shirk, since polytheistic belief is not there. However, it is kufr if one knows that God has forbidden to eat such a food.
Yes. That is why the prophet said that not praying namaz is kufr and that the murderer will go to eternal hell.