The earliest textual sources, including the original manuscript, read a possession rather than simply possession. The use of the indefinite article here is quite odd, which explains why it was omitted in the 1837 edition, most likely on purpose. One possibility here is that the a in 𝓞 is intrusive, that Oliver Cowdery accidentally added it as Joseph Smith dictated the text (or Joseph himself accidentally added it).
The phrase a possession is found elsewhere in the Book of Mormon, but only in 2 Nephi 24:23, where we have a King James quote from Isaiah 14:23: “I will also make it a possession for the bittern”. In this instance, the a is required. But there are instances of this usage in the King James text where the a would be omitted in modern English:
We should also note that elsewhere the earliest Book of Mormon text has a number of unexpected uses of the indefinite article, such as “Alma was a stirring up the people to a rebellion” (Mosiah 18:33), “they were in a preparation to hear the word” (Alma 32:6), and “we had also a plenty of provisions brought unto us” (Alma 57:6). The evidence suggests that the a is intended in all these cases, just as it could be here in Alma 52:26 (see under each of these passages for the evidence).
Besides the one Isaiah quote, there are no other occurrences of a possession in the Book of Mormon text. When the verb is obtain (as here in Alma 52:26), we usually get “obtain possession of X” (that is, without any determiner for possession):
For other verbs, we typically get possession, never a possession:
These many examples suggest the possibility that in Alma 52:26 Oliver Cowdery accidentally inserted the indefinite article a in 𝓞.
There are a number of cases in the text where we get “the possession of ”—that is, where the definite article the occurs even though it is not necessary. In fact, in the following list, there are three examples of “the possession of ” where the verb is obtain (each is marked with an asterisk):
These examples suggest that in Alma 52:26 Joseph Smith could have dictated “obtained the possession of the city Mulek” but that Oliver Cowdery accidentally heard this as “obtained a possession of the city Mulek”. The vowel in a and the in normal speech is the schwa vowel /ß/. The voiced interdental /d/ at the beginning of the could have been obscured by the voiced dental /d/ at the end of obtained, thus leading Oliver to think he had heard a /ß/ rather than the /dß/. For a case where a similar sort of substitution may have occurred, see the discussion under Alma 51:31 (where “with a disappointment of being repulsed” may be an error for “with the disappointment of being repulsed”).
Besides the examples from the King James Bible (listed above), there is evidence elsewhere in Early Modern English for the use of the indefinite article in the expression “a possession of ”, as in the following examples (with accidentals regularized) from Literature Online :
Based on this evidence from English usage, the critical text will restore the original a in Alma 52:26, although there is a possibility that during the early transmission of the text this a was either accidentally inserted or was a mishearing of the.
Summary: Restore in Alma 52:26 the indefinite article a in “Moroni had obtained a possession of the city Mulek”, the reading of the original manuscript; nonetheless, the possibility remains that the original text read without the a (“Moroni had obtained possession of the city Mulek”) or with the instead of a (“Moroni had obtained the possession of the city Mulek”).