The 1858 Wright edition accidentally deleted the coordinating conjunction or, giving a rather diffi- cult (but not impossible) conjunctive structure, providing one interprets the resulting structure as a series of three predicates conjoined by and but explicitly expressed only before the last predicate:
Of course, this interpretation really won’t work because “transgress the commandments of God” and “fall into transgression” are basically synonymous, as can be seen in Jarom 1:10: “if they did not keep the commandments but should fall into transgression / they should be destroyed from off the face of the land” (I owe this example to Don Brugger). Here in Alma 56:21, the original or provides an alternative way of expressing the first predicate. Despite the difficulty of the reading without the or, that shorter reading was copied into the 1874 RLDS edition and not removed from the RLDS text until 1908.
Summary: Maintain the or in Alma 46:21; this or is necessary since it allows for the following predicate to rephrase the preceding one: “if they should transgress the commandments of God or fall into transgression”.