The supralinearly inserted ampersand between cunning and lying appears to be a virtually immediate correction. There is no change in the level of ink flow. Moreover, the and seems necessary if cunning is to be interpreted as a noun. But even with the and, it is possible to interpret cunning as an adjective, especially since the possessive pronoun his is not repeated before the following lying (“his cunning and lying craftiness”). The word cunning would definitely be an adjective if the and were missing from the original text (“his cunning lying craftiness”). On the other hand, if the his were repeated, the interpretation of cunning as a noun would be virtually certain (“his cunning and his lying craftiness”). Thus the corrected text in 𝓟 allows for cunning to be either a noun or an adjective, although the fact that the his is repeated for the third conjunct (“and his fair promises”) but not for the second does suggest that cunning here in Mosiah 10:18 may be an adjective rather than a noun. Of course, the critical text itself does not need to disambiguate the reading here, but for translation purposes the distinction may be necessary.
Elsewhere the Book of Mormon text is nearly evenly divided in its use of the adjective cunning and the noun cunning. There are 12 occurrences of the adjective where cunning directly modifies a noun, as in 1 Nephi 16:38: “he worketh many things by his cunning arts”. On the other hand, there are 11 firm occurrences of the noun cunning, of which six are conjoined with other nouns:
Note, in particular, that the first two listed here (from the book of Mosiah) refer to king Laman, the same person being referred to here in Mosiah 10:18. And both also conjoin cunning with craftiness, just like in Mosiah 10:18. Also note that the first one conjoins cunning and craftiness without any repetition of the determiner the. Thus in Mosiah 10:18, the lack of the determiner his before lying craftiness does not mean that cunning cannot be a noun. In fact, the similar instances of cunning listed above argue that in Mosiah 10:18 cunning should be interpreted as a noun rather than as an adjective, despite the fact that the determiner his is not repeated before lying craftiness.
Summary: Maintain in Mosiah 10:18 the and that separates cunning from the following noun phrase, lying craftiness; here cunning is most likely a noun rather than an adjective.