I respectfully disagree. Even today where the government actually controls the allowable forms of kanji, a (somewhat) free variation persists between the forms. Even in the linked picture above there are characters that would be considered variants today. (In fact in Chinese calligraphy one would use different glyphs when repeating the same hanzi, in order to show one's knowledge.)
Although [亼卩] [亼マ] would be a possible glyphs in kaisho, in gyosho (in which the oldest Manyoshu are written) or sosho it would have been a choice between [亼丶] and [亼マ] . I don't see how one could assert that a particular form is used in the original Manyoshu without having the original text.