Here scribe 2 of 𝓟 initially wrote Abinadi’s name as Aminadi, but he immediately caught his error, crossed out Aminadi (there is no change in the level of ink flow), and then wrote the correct Abinadi inline. Interestingly, this name Aminadi actually occurs three times in the text, in Alma 10:2–3. We should note that here in Mosiah 26:15 was the first time scribe 2 of 𝓟 was required to write down the name Abinadi, so we should not be surprised that he might have initially written the name incorrectly.
The name Abinadi occurs 37 times in the text. The only other place where there is any variation in the spelling of this name is in the book of Alma; in this instance the scribe was once more scribe 2 of 𝓟, and in this case he misspelled this name as Abinedi:
It is possible in Alma 5:11 that 𝓞 itself read Abinedi—that is, the scribe in 𝓞 for that part of the text (probably Oliver Cowdery) accidentally misspelled it there as well. Such a misspelling could have occurred if Joseph Smith pronounced the word with stress on the second syllable so that the third vowel would have been the unstressed schwa vowel (which can be spelled variously in English). In any event, the first occurrences of the name Abinadi are spelled correctly and without variation (although there are no occurrences of the name in the extant portions of 𝓞).
Summary: Maintain the spelling Abinadi for the name of the prophet that confronted king Noah and his priests.