“Before it was possible to stop them in their retreat” can be read two ways. If “them” refers to the Nephites, then they were in such panicked retreat that Mormon could not stop them until they reached Jashon. If “them” means the Lamanites, then the Nephite army could not check the Lamanite invasion until they reached Jashon where they formed a more effective defensive line. I see this second reading as more probable. If the Nephite army had been in panicked flight, massive desertions in all directions would have been the probable outcome, but this did not happen. They stayed together and counter-attacked at Jershon, so successfully that Mormon could go to Antum and retrieve the plates (v. 16; Morm. 1:3).
Geography: Jashon does not appear elsewhere in the record, but it lay somewhere toward the north. Its presence suggests a populated area, not some type of wilderness. It does not fit into previously known names and lands, either because locations have been renamed or because it was new, built as a result of the general population explosion at the beginning of the Classic period about A.D. 250.
Chronology: The 345th year would be A.D. 335. Mormon is probably thirty-four chronologically—hence, in his twenty-fifth year. I deduce this age from the fact that Ammaron had enjoined him to wait until age twenty-four to obtain the plates (Morm. 1:3) and that he, like Joseph Smith, was anxious to receive them at the first possible moment. Of course, Mormon’s movements were under considerable military restraints.