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King's father changed his own name to Martin Luther King, and when he also changed his son's name, the father was Senior and the son Junior, as they had been when the first name was Michael. There are many accounts of why the elder King made these changes (ca. 1934, when Jr. was five years old); see the Wikipedia entries for both men. There's nothing terribly odd about the fact that he didn't legally change the name. He had never really known any other; I suppose he would have had he run into difficulty with it, as he may have had he lived to retirement age. What significance do you attach to it?
Where is the Wiki page that asserts that MLK's use of the "arc of history" quote constitutes plagiarism (which is how we spell that word in English)? Do I commit plagiarism when I say "It's raining cats and dogs"? The expression is not original to me, but I've never bothered to find, still less to cite, its origin.