Here Oliver Cowdery first wrote “save it were” in 𝓟; then somewhat later, with heavier ink, the word save was corrected to except. The supralinearly inserted except looks somewhat like Oliver’s hand, but it is cramped and awkwardly written. The correction may have been made when 𝓟 was proofed against 𝓞.
Elsewhere in the original text, there are 77 occurrences of “save it were” and 14 of “except it were”. Thus either reading is acceptable, which means that there would have been little motivation for editing the text here. It is true that the two preceding instances in the book of Mosiah read “except it were” (in Mosiah 1:4 and Mosiah 3:15), but it is doubtful that these two instances were the source for making the change. A somewhat more likely possibility for the source of the correction in Mosiah 6:2 is the occurrence of the word except rather than save in a very similar passage near the beginning of king Benjamin’s discourse:
Yet it seems rather unlikely that the corrector in Mosiah 6:2 would have hunted back six manuscript pages to recover the except in Mosiah 2:34, especially since save works perfectly well in Mosiah 6:2. The correction is much like several other corrections found earlier in the book of Mosiah that are awkwardly inserted and with heavier ink flow. These corrections may not have been made by Oliver Cowdery, but they all seem to be the result of proofing 𝓟 against 𝓞. (See the discussion regarding the change of with to of in Mosiah 2:13 and the insertion of of in Mosiah 2:32.) Thus the most reasonable assumption here in Mosiah 6:2 is that the correction in the printer’s manuscript represents the reading of the original manuscript, no longer extant for any of the book of Mosiah.
Summary: Retain in Mosiah 6:2 “except it were”, a later correction in 𝓟 that is probably the result of proofing 𝓟 against 𝓞.