In standard English we expect the past-tense subjunctive form had following conditional modals like might and should, which are historically past-tense subjunctive forms. Joseph Smith, in his editing for the 1837 edition for this passage, made the change from the original present-tense hath to the past-tense had. Elsewhere the text uses only present-tense forms of the perfect auxiliary have when referring to the Lord’s having given the land as a land of inheritance:
Of course, all of these examples are found in present-tense contexts, not past-tense conditional contexts. It is possible that in Mosiah 28:2 the hath in 𝓟 is an error for had. But since hath is strange yet not impossible, it seems more reasonable to assume that it is the original reading here in Mosiah 28:2. The critical text will assume as much and restore the difficult but earliest extant reading, “all the land which the Lord their God hath given them”. There is another example where the original text used the present-tense hath in a conditional context; in this instance, the hath was initially changed to had when Oliver Cowdery copied the text from 𝓞 into 𝓟:
Summary: Restore in Mosiah 28:2 the present-tense hath despite its difficulty (“that there should be no more contentions in all the land which the Lord their God hath given them”).