Here the 1874 RLDS edition accidentally replaced the preposition to with unto. The 1908 RLDS edition restored the original to to the RLDS text. The verb return, when referring to returning to a person or to a group of people, can take either to or unto as the preposition. In the original text, there are 13 instances with unto and 11 with to (including the case here in Helaman 5:17). Since either reading is theoretically possible in Helaman 5:17, we follow the earliest textual sources, thus “returned to the Nephites”.
Summary: Maintain the preposition to in Helaman 5:17: “and immediately returned to the Nephites” (the reading in 𝓟, the earliest extant source); either unto or to can be used with the verb return.