Here we have an example of the Hebraistic use of and to separate a main clause from its preceding subordinate clause. For another example involving an if-clause, see 1 Nephi 17:50. And for a complete list, see under hebraisms in volume 3. As with most examples of this Hebrew-like construction, Joseph Smith removed the and in his editing for the 1837 edition.
Summary: Restore in Mosiah 2:21 the original and that connected the subordinate if-clause to the following main clause; such Hebrew-like constructions were apparently part of the original text.