Anyone can speak about Fr. Martin. Anyone can have an opinion about him. But just like opinions about Catholic teaching, they ought to be based on an accurate description of that teaching. The OP, I think, does a very good job of answering some of the typical misunderstandings that some non-Catholics have.
One of the great blessings of my time here at CAF is the be able to hear from knowledgeable Catholics about Catholic teaching. Non-Catholics can have opinions of, say, invocation of the saints, claiming it is idolatry and the line, but when one listens to what good Catholics talk about that topic, an honest person cannot walk away with that misunderstanding, even if one still doesn’t believe in the practice.
Luther’s sin bodly comes from a letter to Melanchthon. In it he is not giving permission to win, even though a brief clip of it might sound like that. Luther is talking about the vastness of grace for the sinner. He uses hyperbole to repay that truth. He doesn’t mention confession/ repentance because it it assumed in a conversation with his colleague.
Read the whole letter. Understand the context of it, understand his writing style, and read other writings that speak to the issue. Finally, remember that it was a letter to a friend, not a doctrinal statement.
Jon