Here the printer’s manuscript reads hath sit. The 1830 typesetter replaced hath sit with hath set, which today would be considered dialectal. The 1920 LDS edition and the 1953 RLDS edition replaced this dialectal hath set with the standard hath sat. For a discussion of the competition in the Book of Mormon text between the intransitive verb sit and the transitive verb set, see under Helaman 14:4.
The question here is whether the perfect construction have sit is possible. Under the list of forms for the verb sit, the Oxford English Dictionary gives examples of standard have sit in the 16th through 18th centuries, as in these two examples from Early Modern English (with accidentals regularized):
The OED also cites dialectal have sit for the 19th century. The critical text will therefore accept the earliest reading here in Moroni 7:27, the archaic (or dialectal) hath sit.
Summary: Restore in Moroni 7:27 the earliest reading, the nonstandard use of sit as the past participle for the verb sit in “and hath sit down on the right hand of God”; such usage occurred in Early Modern English as well as dialectally in the 19th century.