Here in the original manuscript, Oliver Cowdery initially wrote “the possession of the strongest holds”. Virtually immediately he crossed out the plural s, giving “the possession of the strongest hold” (there is no difference in the level of ink flow for the crossout). As explained under Alma 58:23, the Book of Mormon text typically refers to a fortified city as a single strong hold. Thus the corrected reading in 𝓞 (“of the strongest hold”) is undoubtedly the original reading here in Helaman 1:22.
Summary: Maintain in Helaman 1:22 the singular number for strongest hold, the corrected reading in 𝓞; elsewhere the text typically refers to a single city as a strong hold.