One wonders if the text here isn’t missing a what after but. The original manuscript is extant for this passage and there is no what inserted there. Elsewhere in the text we have three examples of “know not but what”:
There is also one example of “know not but that”:
Despite these examples, there is also an example without either the what or the that, and the original manuscript is extant for this passage:
Although the expression “know not but” without the what seems strange in 1 Nephi 17:43 and Alma 56:43, it does appear to be possible in the Book of Mormon text.
Summary: Maintain the expression “know not but” without a following what or that in 1 Nephi 17:43 and Alma 56:43; both readings are supported by the original manuscript.