There are two scribal corrections here in 𝓟. In the first case, Oliver Cowdery initially skipped the first relative pronoun which, but then almost immediately he supralinearly inserted it (the level of ink flow is unchanged). The which is necessary since its antecedent, the seats, acts as the direct object of the verb ornament found later on in the sentence. Yet the virtual immediacy of the correction shows that the supplying of the which was not due to editing; instead, Oliver was simply making sure that 𝓟 followed the reading in 𝓞.
For the second correction, Oliver Cowdery crossed out the fine that he had initially written and then supralinearly inserted pure. The correction is with somewhat heavier ink flow and may be nearly immediate, with Oliver having redipped his quill before making the correction; another possibility is that this correction was done a little later as Oliver was proofing 𝓟 against 𝓞. There are two instances of pure gold and two of fine gold elsewhere in the Book of Mormon:
(The 2 Nephi 23:12 example quotes Isaiah 13:12.) Thus either fine gold or pure gold is possible in Mosiah 11:11, which suggests that the change of fine to pure in 𝓟 was not the result of editing.
Summary: Retain in Mosiah 11:11 the two corrections Oliver Cowdery made in 𝓟: namely, the insertion of the relative pronoun which and the change of fine gold to pure gold.