The question here is whether the original singular land in 𝓞 was changed to the plural lands by inserting an s inline. Oliver Cowdery started to write around (at least the first two letters, ar), then erased the ar, overwrote it with ro, and then finished the word by writing the final und inline. As a result of the erasure, 𝓞 is difficult to read here; and one wonders whether the noise after the d of land is an inserted s or the effects of the erasure of the ar. In any event, Oliver Cowdery copied this word into 𝓟 as the singular land, which has been retained in all the printed editions. (For discussion of the tendency in 𝓞 for Oliver to write around about rather than round about, see under 1 Nephi 8:13.)
Elsewhere in the text, we normally have the singular land for the expression “(all) the land(s) round about” (13 times), including the following three cases where the expression refers to the land surrounding a specific land that has just been mentioned in the text:
This is precisely how the text reads here in Alma 50:9; even the preposition is repeated (in this case, in): “in the land of Zarahemla and in the land round about”. There is only one place where the earliest text reads “the lands round about”, and in that instance the phrase is not used to refer to the land surrounding a just-mentioned specific land: “the disciples of Jesus had formed a church of Christ in all the lands round about” (4 Nephi 1:1). Thus the most reasonable assumption is that Alma 50:9 originally read “and in the land round about”.
Summary: Maintain the singular land in Alma 50:9, the apparent reading in 𝓞 and the firm reading in 𝓟 and all the printed editions; the singular usage is consistent with three other examples in the text where “the land round about” refers to the region surrounding a specific land; also maintain the form round rather than around in this sentence.