We don't have synonyms for mercy seats yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are mercy seat and mercy-seat.
(n) :
(Judaism, Christianity) The lid of the Ark of the Covenant, on which God is said to have been enthroned; taken to represent the throne in heaven.
(n) :
(Judaism, Christianity) The lid of the Ark of the Covenant, on which God is said to have been enthroned; taken to represent the throne in heaven.