Here we have two more examples of the mixed gerundive construction (namely, “the fulfilling the commandments”), mixed in the sense that the gerundive is both verbal and nominal. In today’s English we expect either the first the to be lacking (thus the more verbal form, “fulfilling the commandments”) or an of to occur after fulfilling (thus the more nominal form, “the fulfilling of the commandments”). The fact that here in Moroni 8:25 we get two occurrences close together of the same mixed construction strongly argues that this mixed gerundive form is acceptable style for the Book of Mormon. For further discussion, plus a number of examples, see under 1 Nephi 17:32. There is also evidence for this kind of mixed gerundive construction in the English of Benjamin Franklin, as in the following example cited on page 415 of The Works of Benjamin Franklin, edited by Jared Sparks, volume 10 (Boston: Hilliard, Gray, and Company, 1840):
For additional examples from Franklin as well as from others, see under gerundives in volume 3; also see under Alma 55:19.
Summary: Maintain in Moroni 8:25 the two instances of the mixed gerundive construction, “the fulfilling the commandments”, the reading of the earliest text; such usage can be found elsewhere in the Book of Mormon and in earlier English.