He has taken every private thing. Willie has made confessions, what for? He has taken everything out of the way that other hearts that were there might have the example to clear the way, and now they have taken advantage of all that. "He has admitted," they say," that he has manipulated this." But to manipulate means to change, to alter. There is not a thing of it, because he brings it right to me, and I know there is not a thing changed or altered. But sometimes he thinks that some part -- they might be in a better position if we rush it out upon them at once. Perhaps they will get into a soft mood, that they will be likely to give heed to the testimonies. But as to manipulating and changing thm, I know better. That is false. Because I have the last reading of everything that goes out. There is not anything that is manipulated, as they call it, taken from or added to it, that I know of. There is no such thing. He frequently changes a word for another word meaning the same, but that may seem not so hard--a word expressing the same thing, but still they might feel a little bitter over it. Ms 136 1906

There are other places showing that this was a big issue in 1906. Below, I explain how I currently understand it, with the object to harmonize as much as possible, because I do believe fully that Ellen White was inspired.

In 1906, the problem with Kellogg and A.T. Jones and others in that group was coming to a head. They had nearly gained control of the medical arm of the SDA Church, and were trying to get the spiritual arm too. Of course if they did that, the SDA Church would basically turn into New Age goofiness, and ravel out like Satan planned. There were a few top leaders facing them, like Daniells and Haskell, but Ellen White was the main one, so they introduced doubts into peoples' minds regarding the trustworthiness of her writings. They knew that Willie White would change some words around here and there, and he admitted as much. So they capitalized on that admission, trying to make it appear that all of Ellen White's writings were untrustworthy. This is the issue Ellen White is addressing in this Manuscript 136, and in a number of others in this same time period. If her voice would be effectively silenced, the Kellogg crew could basically take over the entire SDA Church. But God wouldn't have it, and sent message after message to Ellen White to thwart their purposes. 

So while true that Wille White did not manipulate his mother's writings in the sense that he changed them willy-nilly, the fact remains that he did change some words here and there.