In the printer’s manuscript, Oliver Cowdery wrote this name with a space between Morian and cumer. The c, however, was not capitalized, which argues that Oliver intended to write Moriancumer as a single unit. Nor did he later insert a hyphen between the two parts. The 1830 compositor’s decision to set Moriancumer as one word is probably correct.
Generally speaking, Book of Mormon names (excluding the biblical names in the Isaiah quotations) are spelled as single units (with no hyphens or spaces intervening). The only exceptions appear to be names that refer to places or to peoples: Ani-Anti, Anti-Nephi-Lehi, Lehi-Nephi, and Jacob-Ugath, none of which are names of persons, at least originally. (As explained in Alma 24:3, the brother of king Lamoni took on the name of his converted people, Anti-Nephi-Lehi, when he became king over all the Lamanites.)
Another factor to consider here is that Moriancumer could be a misspelling for Moriancumr (that is, without any explicit vowel in the last syllable). When Oliver Cowdery initially spelled the name Coriantumr in Helaman 1:15, he spelled it phonetically, as Coriantummer. So one wonders here in Ether 2:13 whether the scribe in 𝓞 (presumably Oliver) once more made the mistake of adding a vowel for a name ending in mr but this time the error was not corrected. The critical text, however, accepts minor differences in names that refer to different individuals. For a good example of this, see the discussion regarding the name Ammaron under 4 Nephi 1:47.
The brother of Jared is sometimes referred to by the name Mahonri Moriancumer, but this name is extracanonical. George Reynolds, in a footnote on page 282 of his article “The Jaredites” in The Juvenile Instructor 27/9 (1 May 1892), discussed the origin of this name (original punctuation retained):
While residing in Kirtland Elder Reynolds Cahoon had a son born to him. One day when President Joseph Smith was passing his door he called the Prophet in and asked him to bless and name the baby. Joseph did so and gave the boy the name of Mahonri Moriancumer. When he had finished the blessing he laid the child on the bed, and turning to Elder Cahoon he said, the name I have given your son is the name of the brother of Jared; the Lord has just shown [or revealed] it to me. Elder William F. Cahoon, who was standing near heard the Prophet make this statement to his father; and this was the first time the name of the brother of Jared was known in the Church in this dispensation.
(The bracketed or revealed is in the original.) The ultimate source for this account is William F. Cahoon, an older son, who was there. Joseph Smith gave the name to the seventh son of Reynolds Cahoon and Thirza Stiles Cahoon; he was born in 1834 in Kirtland, Ohio. (I wish to thank Ken Cahoon for this additional information from the Cahoon family records.) Apparently the family itself determined the spelling for Mahonri but followed the Book of Mormon spelling for Moriancumer.
Summary: Accept in Ether 2:13 the spelling Moriancumer for the place where the Jaredites encamped on the seashore; the name is spelled as two words in 𝓟, but this appears to be a scribal error; there is a possibility that the actual spelling of the name was Moriancumr, but this can be only indirectly deduced from the name Coriantumr.