As discussed earlier under Mosiah 23:13–14, sometimes sentence-final present participial clauses are separated from the preceding part of the sentence by the conjunction and. Here is one of these examples; for another example in this chapter, see Mosiah 28:4. As in other instances, the initial and before the participial commanding implies the notion of ‘also’ (see the discussion under Mosiah 28:4).
Here in Mosiah 28:20, Joseph Smith removed this instance of the separated present participial clause by changing the nonfinite verb form commanding to the finite commanded. The critical text will, of course, restore the original “and commanding him that he should keep and preserve them”. Another possible emendation would have been to delete the and, which is what the committee for the 1920 LDS edition did when they edited Mosiah 28:4 (see there). But in most cases, editing has replaced the present participle with a finite verb form (see the examples listed under Mosiah 23:13–14).
Summary: Restore in Mosiah 28:20 the original present participial clause; also maintain the conjunction and that detaches this nonfinite clause from the preceding finite clause (“and conferred them upon him and commanding him that he should keep and preserve them”); such usage can be found elsewhere in the text.