The a that occurs before most in the earliest textual source for Ether 8:18 (the printer’s manuscript) was also probably in the original manuscript (there is room for it in the lacuna between extant fragments). This a was, however, probably an error; perhaps Oliver Cowdery expected to write a noun as the subject complement for combination (something like “which combination is a most abominable and wicked one”). Or he may have been influenced by the occurrence of a in the preceding “they formed a secret combination”. Ultimately, what seems to have happened is that Oliver never crossed out the intrusive a in 𝓞, even though he might have intended to.
Elsewhere in the text there are no examples of “most wicked”, but there are several of “most abominable”. Yet this adjective phrase occurs only as a subject complement (that is, in the predicative position), never in the attributive position premodifying a head noun. And in each case of “most abominable”, the following text involves a statement saying that something is “(the) most abominable” with respect to all (other) relevant possibilities:
The second instance (in 1 Nephi 13:26), with its definite article the, involves ellipsis of its noun, as if the text read “which is the most abominable church of all other churches”. In any event, the occurrence of the indefinite article a in “a most abominable and wicked above all”, the earliest text for Ether 8:18, seems impossible. Thus Joseph Smith’s removal of the intrusive a in Ether 8:18 was most likely correct, and the critical text will accept his emendation as the original reading for this passage. (For an example where the indefinite article an was incorrectly inserted before the noun abomination, see under Jacob 2:28.)
Summary: Accept in Ether 8:18 Joseph Smith’s 1837 emendation that removed the intrusive a before most abominable; Oliver Cowdery apparently miswrote “a most abominable” in 𝓞, which was then copied into 𝓟 and into the 1830 edition.