Here in the original text we have another instance of the Hebraistic and separating an initial subordinate clause from its following main clause (“when they cast their eyes about … and behold they saw that … ”). The 1920 LDS edition removed the unexpected occurrence of the and here. The critical text will, as explained under 1 Nephi 4:8–9, restore or maintain (depending on the case) each original instance of and that separates a preceding when-clause from its following main clause. For a complete discussion of this Hebraistic construction, see under hebraisms in volume 3.
Summary: Restore in Helaman 5:43 the original Hebraistic use of and that separates the preceding when-clause from its following main clause; the original text has numerous examples of this kind of subordinate construction.