Scribe 2 of 𝓟 consistently wrote Nephihah as Nephiah, not only here in Alma 4:17 but also three other times. Oliver Cowdery, in each of the cases, corrected the spelling by inserting the missing middle h; the three other examples are also found in the first part of Alma, where scribe 2 was the scribe in 𝓟:
It is possible that scribe 2’s error was influenced by common biblical names that end in iah, such as Isaiah and Jeremiah. Since either spelling, Nephihah or Nephiah, is theoretically possible, Oliver Cowdery’s four corrections to Nephihah undoubtedly represent the reading in 𝓞.
Elsewhere in the text (Alma 50–62), there are 20 other occurrences of the name Nephihah— and all with that spelling. Oliver Cowdery is the scribe in both manuscripts for these later uses of the name. Of these occurrences, eight of them are extant in the original manuscript and all eight are spelled with the middle h. Thus the earliest extant readings for Nephihah, in the original manuscript, are all spelled that way.
Morphologically, we have several pairs of Book of Mormon names that support the use of -hah as a distinct morpheme at the end of names. In addition to Nephi /Nephihah, we have Cumeni /Cumenihah, Mathoni /Mathonihah, and Moroni /Moronihah. Thus evidence from Book of Mormon names suggests that Nephihah is derived from Nephi by adding -hah. Of course, the manuscript evidence itself strongly supports Nephihah as the original name.
Summary: Retain Oliver Cowdery’s spelling Nephihah in Alma 4–8 (namely, his correction in 𝓟 of scribe 2’s consistently misspelled Nephiah); Nephihah is supported by all the extant spellings of this name in the original manuscript.