In verse 5 the original and printer’s manuscripts read “in the which things I do rejoice”. If we include text from the previous verse, one can interpret the passage as telling us that Lehi rejoiced in two things: he has not perished and he has obtained a land of promise. The land is only promised, of course; Lehi hasn’t made it there yet. For a discussion of this kind of language as an example of the “prophetic perfect”, see pages 164–166 in Donald W. Parry, “Hebraisms and Other Ancient Peculiarities in the Book of Mormon”, Echoes and Evidences of the Book of Mormon, edited by Donald W. Parry, Daniel C. Peterson, and John W. Welch (Provo, Utah: Foundation for Ancient Research and Mormon Studies, Brigham Young University, 2002), 155–189.
The 1830 edition changed the plural things to the singular thing, probably because the relative pronoun which is readily interpreted as applying to only the most recent clause (namely, that Lehi has obtained a land of promise). Despite the easiness of the 1830 reading, the plural was restored in the 1837 edition, probably by reference to 𝓟.
We have no examples of scribe 3 of 𝓞 mistakenly adding the plural s (although there are a number of examples where he did drop it). So there is no direct scribal evidence from scribe 3 to suggest that the plural s is an error in 𝓞.
Elsewhere in the text there are three examples of “which thing” and two of “which things”:
The last example is particularly interesting since it refers to two actions, the rebuilding of the old Jerusalem and the building of the new Jerusalem. Thus the plural “which things” is appropriate there. We also note that both 1 Nephi 5:5 and Ether 13:6 have the same type of archaic prepositional construction (“in the which things” and “for the which things”).
Summary: Retain the plural usage “in the which things I do rejoice”; there is no direct scribal evidence to suggest that things is incorrect in 1 Nephi 5:5; the plural reading is supported by the larger passage as well as by the same usage in Ether 13:6.