The phrase “yea and” is very frequent in the Book of Mormon text. There are, for instance, quite a few cases of “yea and I” followed by some form of the verb know:
Thus there is nothing unusual about the original occurrence of “yea and I know” in 2 Nephi 9:5. The dropping out of the and in the 1837 edition appears to be simply a typo and not due to Joseph Smith’s editing.
Summary: Restore the earliest reading “yea and” in 2 Nephi 9:5 (“yea and I know that ye know that in the body he shall shew himself unto they at Jerusalem”).