Alma now repeats the reference to the Nephite foundational promise that he saw in Alma 36:1. If “ye will keep my commandments ye shall prosper in the land…” English does not make an easy distinction between the singular “you” and the plural “you.” Many other languages do, and we may suppose that the language of the Nephites did. It is most likely that in the Nephi and Lehi presentation of this promise, the “ye/you” was a plural subject. The reference would be to the entire people, not a single person.
This was a national promise, not an individual promise. It would appear in this verse that Alma has personalized it. I would suggest that we have in this verse the singular “you,” and that he is personalizing this promise directly to Helaman. Alma has just described the tremendous importance of the records (how much more important can something be than to have God directly interested in it?). Now he repeats the formulation of righteousness and prospering, but he adds a contrary phrase.
In the original formulation of the promise, the result of not being righteous is implied, not stated. Of course the implication is that if prospering comes with righteousness, that unrighteousness would lead to a lack of prospering. In this case, the explicit result is not a lack of prospering, but rather being cut off from the presence of the Lord. While that is certainly contrary to a notion of prospering, it is a tremendous threat for one who is to become the prophet of a people. His personal righteousness will allow him to prosper, but the alternative is that he will be cut off from the Lord, he will be no prophet at all.
As a side note, we should note that “prospering” may have a more mundane context than we typically assume. We assume that we prosper when all goes well. Helaman will be a righteous man, but all will not go well for him. The “prospering” in this case is a continued existence – a permanence of presence. Helaman will “prosper” in that he will be preserved through the coming wars.