Here the 1874 RLDS edition and the 1906 LDS large-print edition independently replaced the singular possession with the plural possessions. Such changes show that editors or typesetters expect the full plural expression “the lands of X’s (own) possessions”, as in six places elsewhere in the text (see the list under 2 Nephi 29:14). On the other hand, there is evidence of a tendency to replace the plural possessions with the singular in this expression:
So one could argue that the singular possession here in Helaman 5:52 is an early error for the correct possessions.
One example that opposes this change is the nearby example in Helaman 7:22, which reads without variation as “the land of our possession” (but which is not extant in 𝓞). In addition, as discussed under Helaman 4:8, there are instances in the original text of possession where English speakers expect possessions. For this reason, the critical text will accept the singular possession here in Helaman 5:52, the reading of the earliest extant textual sources (𝓟 and all the early editions).
Summary: Retain in Helaman 5:52 the singular possession in the phrase “the lands of their possession”; although speakers may expect the plural here, the singular can be found elsewhere in the text.