One need only watch the movie "Gandhi" to know that the British were extremely violent in India during the Gandhi era. They have a vicious history in Ireland. The only "colony" that the British were not extremely violent with was during our Revolutionary War -- though they were violent to an extent, cooler heads there recognized that they could still exploit our nation's natural resources by way of trade. They simply loosened the grip on taxes.
Actually, King did not change his views later in life. There is zero evidence of that. He did recognize that his movement was not as effective in the north, and that a young generation did not see why they should be nonviolent at home while being violent in Vietnam. King did tell associates that he was frustrated in that he recognized there would be a period of violence -- but as his Poor Peoples Campaign plans demonstrate beyond debate, he was not giving up.
Indeed in the last year of Malcolm's life, the two communicated through a lawyer in Chicago. Malcolm advocated expanding the struggle -- including in the UN -- as a result of his travels. One need to look no further than Martin's greatest speech, exactly a year to the day before he was killed, to see that he had expanded from civil rights. (Any student worth their salt can trace King's writings and speeches from, say, 1966 on to see the direction he was headed in.
I should note that Dr. King had kept a shotgun in his house early in his ministry, but did get rid of it as he progressed.
My late friend Rubin knew and was friends with Malcolm, Martin, and Coretta -- and eventually Ali -- so I think my insights on that era, rooted in knowing Rubin for 40+ years (including working with him on the book the top quote of my OP comes from), classes in state universities, and an extensive collection of books both by and about Malcolm and Martin -- and even LPs of their speeches from a Detroit record company in the '60s -- provides me a background at least slightly thicker than a bumper sticker.
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