The problem here is whether the comparative longer can occur without any or some other marker of negation. Elsewhere in the text, we have 12 occurrences of “no longer” and 5 of “any longer”. For each of the 5 cases with any, there is a preceding not in the clause. In all 17 cases of nonassertive longer, there has been no textual variation involving the loss of no, not, or any. Interestingly, the example of longer here in Mormon 9:3 is the only case where the word occurs in a question rather than in a statement or command; the use here without any negative marker is unique for the Book of Mormon text.
However, this usage is found in a revelation to Joseph Smith given in March 1830, less than a year after the completion of the Book of Mormon translation:
This example also takes the form of a yes-no question, just like in Mormon 9:3, thus providing support for the use of longer without any negative marker (no, not, or any). The phraseology in Mormon 9:3 will therefore not be emended, even though it sounds strange to modern English readers.
Summary: Maintain the reading in Mormon 9:3 where longer occurs without a negative marker in a yes-no question; such usage is unexpected but is apparently intended.