With regards to the question as to whether Luther believed that each individual has an assigned guardian angel, from his comments on the subject that I've read, he held that not just one angel looks after each individual but that many do, but whether he believed they're specially assigned to each individual is harder to assess. He held that different ranks of angels are assigned to different ranks of people but that's not quite the same. Here's an excerpt from his sermon on Matthew 18:1-10 preached on The Day of St. Michael and all Angels in 1532:
What we should learn today is that our Lord God has so ordained that each Christian should have not only one but many angels protecting him, just as each also has his own particular devils who slink after him and, if unable to do more, trouble a person with bad dreams, saddle you with scandalmongers who slander, thwart, and defame you. Be aware that these are all wicked henchmen of the devil and be careful that you do not live as do the heathen; they, too, see and experience such misfortune, but in their thinking it just happens accidentally, for they know nothing about the devil. But let us learn to recognize that this is the accursed devil's doing. On the other hand, when you see that something has turned out well, be ready to say that a good angel prompted it, otherwise things would have gone worse for a person. To illustrate, when a person is saved from a flood or is unharmed when a stone falls on him, this is not luck but the work of a beloved angel.
But there are distinctions among angels, as well as among the devils. Princes and lords have great and special angels, as we see in Daniel 10. Children and common folks have ordinary angels. For there will always be a given angel who is greater, mightier, and wiser than the rest. The same is true, also, of the devils. That explains why, as Christ says, that they belong to a kingdom (Luke 11). For just as many persons and different offices constitute a kingdom, so also among the devils there are different ranks. There are the lesser devils who tempt with adultery, greed, vain arrogance, and other similar sins. But then there are higher spirits who tempt with unbelief, with despair, with heresies, and have misled the factious spirits and the pope. It is extremely important, therefore, that we accustom ourselves to prayer, and in time of grave danger conscientiously petition God not to take from us the protection of the beloved angels, lest we come to nought, for we are too weak for the devil. (pages 380,381; The Complete Sermons of Martin Luther, Volume 7, published by Baker Book House, 2007)
I need to add something to my answer above. I've come across a statement Luther made two years later in 1534, when he preached on the same feast day, where he says the following:
Also, we are to know for a certainty that every human being has his own appointed angel who looks after and watches over him. We are familiar with the common proverb that expresses people's customary comment when someone is shielded from harm, You had a good angel! or, Your holy angel was with you and protected you. That is well said and, in fact, nicely reminds us of the benefaction God bestows on us through his beloved angels. (page 387, ibid)
So Luther did believe we each have our own guardian angel.