Kashrut

If a piece of liver was frozen for weeks, can it be cooked and roasted?

Is there timtum halev [approximately, spiritual pollution of the heart] when one ingests non-kosher food in a halachically valid manner, e.g., based on bitul (nullification)?

Reuven sold supposedly kosher wine on credit to Shimon, who had a retail business. Shimon sold it to several customers. Later on, it became clear based on witnesses and Reuven’s admission that it was stam yeinam (wine handled by non-Jews, which is Rabbinically forbidden). To what extent does Shimon have to pay Reuven for the wine that Shimon already sold? What should be the arrangement be between Shimon and the people whom he accidentally caused to drink non-kosher wine?