The original manuscript, although extant here, is difficult to read. The preposition seems to be into, although unto is also possible. The word is definitely not the shorter to. In any event, Oliver Cowdery wrote into in the printer’s manuscript when he copied the text from 𝓞 into 𝓟. In the 1840 edition, the into was replaced by to. This change is probably a typo rather than the result of conscious editing. Finally, into was restored to the RLDS text in 1908.
Elsewhere in the text, we have three occurrences of “marching into a land” and four of “marching to a land”, but none of “marching unto a land”:
In the case of Helaman 1:17, 𝓞 is extant and reads to; in 𝓟, however, Oliver Cowdery initially wrote into but then virtually immediately corrected it to to. In each case, we rely on the earliest textual sources to determine the preposition. Thus the reading in Alma 52:2 is most probably “into the land northward”, not “unto the land northward” (nor “to the land northward”).
Summary: Accept in Alma 52:2 the preposition into in the phrase “into the land northward”, the probable reading in 𝓞 and the definite reading in 𝓟.