Here Oliver Cowdery initially wrote “except ye repent” in the printer’s manuscript; then virtually immediately he inserted the modal verb shall before repent (the level of ink flow is unchanged). Undoubtedly, “except ye shall repent” was the reading of the original manuscript. For a list of other cases where Oliver omitted the modal shall, sometimes only momentarily, see under 1 Nephi 17:46.
Elsewhere in the original text, there are 18 occurrences of “except ye repent” and 5 more of “except ye shall repent”. It is not surprising here in Helaman 9:22 that Oliver Cowdery initially wrote the more common expression (“except ye repent”). But the other is also possible, so there definitely would not have been any grammatical motivation for Oliver to have edited the text towards the less common expression. For another case where the shall has been omitted from the phrase “except ye shall repent”, see under Jacob 3:3.
Summary: Accept in Helaman 9:22 Oliver Cowdery’s correction in 𝓟 from “except ye repent” to the less common “except ye shall repent”, undoubtedly the reading in 𝓞 (which is not extant here).