Lyle Fletcher (personal communication, 22 September 2004) suggests that the an is intrusive here in Ether 9:26; that is, the original text read “there began again to be exceeding great wickedness upon the face of the land”. Elsewhere in the text there are examples of wickedness with various determiners (including the case of no determiner), but there are no others that take the indefinite article a /an:
The use of the indefinite article here in Ether 9:26 suggests a particular wickedness rather than wickedness in general. And in fact, this is what the verse appears to be referring to. Note that right after this reference to “an exceeding great wickedness”, the text refers to the rise, once more, of secret combinations among the people: “and Heth began to embrace the secret plans again of old / to destroy his father” (Ether 9:26). Thus the use of the indefinite article an earlier in the verse could be fully intended since secret combinations would constitute an especially evil wickedness. In addition, there is no scribal evidence for accidentally inserting an in front of exceeding.
More generally, there are two cases where an seems to have been consciously added to a noun phrase (and in both cases, by the 1830 typesetter):
And in one other case, an seems to have been accidentally added (in the 1840 edition) because the following word, ancient, began with an:
But here in Ether 9:26 there seems to have been no motivation for adding the an before exceeding. The critical text will therefore accept the reading here in Ether 9:26 of “an exceeding great wickedness”; here we apparently have a reference to secret combinations.
Summary: Maintain in Ether 9:26 the use of an before exceeding, the reading of all the textual sources; here the text seems to be referring to the particular wickedness of secret combinations.