Although this change was introduced into the 1840 edition, it is probably a typo and not due to Joseph Smith’s editing for that edition. There is no good grammatical reason to delete the and here, nor did Joseph do this kind of editing for the 1840 edition (or for the 1837 edition). The 1908 RLDS edition restored the and, undoubtedly by reference to the printer’s manuscript. The LDS text, which derives from the 1837 edition through the 1841 British edition, has always maintained the and.
Summary: Maintain the conjunction and in 2 Nephi 9:51, the original (and current) reading of the text.