One wonders here whether the singular all the possession might be an error for all the possessions. There is evidence in the manuscripts that Oliver Cowdery occasionally mixed up the grammatical number for possession(s); for examples, see under Alma 22:33–34. Here in Helaman 4:8, the occurrence of the singular was in the following relative clause (“which was in the land southward”) is not evidence against the plural possessions since such cases of subject-verb disagreement were prevalent in the original text (see the general discussion under 1 Nephi 4:4).
When we consider other instances of possession(s) in the text, we find that either singular or plural can occur. Consider, for instance, the phrase “the land(s) of one’s possession(s)”, which is discussed under 2 Nephi 29:14: although in the earliest text there are six instances of “the land(s) of one’s possessions”, there are also two instances of “the land(s) of one’s possession” (in Helaman 5:52 and Helaman 7:22). Similarly, we get a choice in number for the phrase “no more possession(s)”, with one instance in the singular and two in the plural (see the discussion under Alma 22:33–34). Moreover, there are other instances of plural possessions involving lands that support emending possession to the plural in Helaman 4:8:
Despite these examples, the singular possession is not impossible here in Helaman 4:8; in fact, there has never been any tendency in the textual history to change this instance of possession to the plural, which argues that the singular is not all that difficult. The critical text will therefore continue with the singular possession here, even though it could be an error for possessions.
Summary: Retain in Helaman 4:8 the singular possession despite its unusualness in the expression “all the possession of the Nephites which was in the land southward”; usage elsewhere in the text generally allows for either singular or plural for the word possession(s).