Here Oliver Cowdery originally wrote an ampersand in the printer’s manuscript; then somewhat later he edited this and to or, probably while proofing against the original manuscript. The ink flow for the supralinearly inserted or is heavier, which argues that the change was not immediate.
The use of and implies that there is only one condition here, with the result that not knowing the will of God leads to sinning ignorantly (this is a possible interpretation). Actually, the use of or could also imply the same meaning for both relative clause conjuncts since in the Book of Mormon the conjunction or is frequently used to correct or explain the preceding text. On the other hand, the conjunction or may introduce an alternative condition. One could interpret this passage as first referring to those who never knew the will of God at all and second to those who knew something of right and wrong but who never received any specific commandments from the Lord, so they “ignorantly sinned”.
Either conjunction (and, or) seems acceptable; since there appears to be no motivation for replacing one with the other, the corrected reading in 𝓟 is probably just a correction based on 𝓞 (no longer extant here). There are a number of other passages where relative clauses are conjoined with an or (here cited according to the original text):
Note that the first two examples listed above are a part of king Benjamin’s address; thus the occurrence of or who earlier in Mosiah 3:11 is consistent with this usage.
Summary: Retain in Mosiah 3:11 the conjunction or in “who hath died not knowing the will of God concerning them or who have ignorantly sinned”; Oliver Cowdery’s correction here in 𝓟 was probably based on the reading in 𝓞, no longer extant here.