Luther had the same problem as Augustine: he lived too long and wrote too much. Each solved the problem in accordance with his personality. Augustine very systematically reviewed his works and published the Retractiones. Luther simply put forth a quip in the introduction the Jena collection of his works: that he wished only his Catechism for children and the De Servo Arbitrio would survive.
This is why I'm so down on ever trying to "prove" a doctrine with quotations from Luther. Which Luther are you quoting? The author of the Small Catechism or the author of the Babylonian Captivity or the author of Against the Jews and Their Lies or the guy sitting at his dining room table holding forth for the students or the guy yelling at the congregation in Wittenberg that he is "your bishop"?
This does not diminish the genius of Luther or his status as the chief theologian of the Augsburg Confession. It should just make us cautious. Luther must be approached not as a source of proof texts but rather as 1) a source of thought-provoking theological discussions and 2) a source for telling us just what Luther was thinking on a given topic on a given day. To ask more of the works of Luther that are not referenced by name in the Confessions is asking too much.
To that end, here are two Luther quotations on topics that continue to be...unresolved. Again, I don't see either as a proof text. They are simply interesting data to add to the discussion. One is from the Table Talk and deals with questions of receptionism and reservation. The other is from the marginal note of Luther's 1545 Bible at Matthew 1:25 and deals with the semper virgo. Thanks to various friends for passing them on.
From the Table Talk:
When the doctor [Martin Luther] was asked whether the sacrament can be carried to the sick, he replied, “We don’t think it should be done. To be sure, one must allow it for a while. The practice will probably be dropped, if only because they have no ciborium." What should be done about it? In our churches, too, there’s debate about whether the [elements of the] sacrament should be carried to another altar for consecration. I put up with it on account of several heretics who must be opposed, for there are some who allow that it’s a sacrament only while it’s in use; what is left over and remains they throw away. That isn’t right. We let somebody consume it. One must never be so precise [and say that the sacrament remains a sacrament when carried] four or five steps or when kept so-and-so many hours. What does it matter? How can one bless the bread for each and every one? We also retain the practice of elevating the sacrament on account of several heretics who say it must be done so. It must not be done so, for as long as one is engaged in the action even if it extends for an hour or two or even if one carries it to another altar or, as you do” (he said this to Cordatus), “across the street, it is and remains the body of Christ.”
Marginal Note at Matthew 1:25 in Luther's Bible (1545):
It should not be understood that Joseph knew Mary afterwards. Rather this is a manner of speaking in the Scriptures. As in Genesis 8, the raven did not come back “until the soil dried out;” the Scripture does not mean that the raven came back after that. So also it does not mean to say that Joseph knew Mary afterwards.