Here Oliver Cowdery first wrote “at that time will the day come that …”, but later (probably when he read back the text to Joseph Smith) he crossed out the original will before the subject the day and then supralinearly inserted it after the subject, all with somewhat heavier ink flow (which shows the change was not that immediate). Since either word order is acceptable, this change does not appear to be editing on the part of Oliver or Joseph; instead it seems to represent their attempt to get the text down correctly.
This passage is the only example in the Book of Mormon text where the adverbial “at that time” appears at the beginning of a clause. So we have no evidence elsewhere in the Book of Mormon text as to whether we should have the noninverted word order of modern English for this adverbial (“then at that time the day will come”) or the inverted word order of earlier English (“then at that time will the day come”). In the King James Bible, we have examples of both types of word order after a clause-initial “at that time”. The noninverted modern word order is more frequent (20 versus 8). One interesting example of this contrast is found in the following King James passage:
With other sentence-initial adverbial phrases, we get either word order in the Book of Mormon, as in the following pair of examples involving “at that day” and the modal verb shall:
noninverted word order: adverbial + subject + auxiliary verb + main verb
inverted word order: adverbial + auxiliary verb + subject + main verb
Since either word order is possible in the Book of Mormon, the critical text will follow Oliver Cowdery’s corrected word order in the original manuscript for 2 Nephi 25:16 (“and then at that time the day will come”).
Summary: Retain in 2 Nephi 25:16 Oliver Cowdery’s corrected word order in 𝓞: “then at that time the day will come”.