Here in Mosiah 11:9, one may wonder if the preposition with was accidentally omitted before the conjunct silver during the early transmission of the text. There is considerable evidence that the preposition is almost always repeated in conjuncts of gold and silver (for discussion, see under Jacob 2:12). On the other hand, there is considerable evidence that for the specific preposition with, repetition is not that consistent (for discussion, see under 2 Nephi 27:2). Thus it is probably safest in Mosiah 11:9 to assume that the earliest extant reading is the original reading. Generally speaking, the critical text will follow the earliest textual sources for each potential case of prepositional repetition. For a complete discussion regarding the tendency to accidentally omit the repeated preposition, see conjunctive repetition in volume 3.
Summary: Maintain in Mosiah 11:9 the earliest reading which lacks the repeated with for silver (“and was ornamented with gold and silver and with precious things”); usage elsewhere argues that such a reading is possible, although it is also possible that with was accidentally omitted here in the early transmission of the text.