Here the first RLDS edition (1874) accidentally replaced the preposition of with at, probably because of the use of “at Jerusalem” in the previous verse, especially given the parallelism between the two questions:
The reading with at in verse 19 has continued in all subsequent RLDS editions.
The correct preposition is of, the reading of both 𝓟 and the 1830 edition. Such usage is also consistent with all other usage in the text: there are 41 other occurrences of the specific phrase “the land of Jerusalem” but none of “the land at Jerusalem”. As explained under Alma 7:10, the text does not at all avoid the unexpected reading “the land of Jerusalem”.
Summary: The correct reading throughout the text is “the land of Jerusalem”, including Helaman 16:19 (“as well as in the land of Jerusalem”).