In the last clause of this verse, the printer’s manuscript reads “his mysteries and his miraculous power”, but the 1830 typesetter changed miraculous to marvelous, probably because earlier in the verse the text refers to “his mysteries and marvelous power”. The 1908 RLDS text restored the word miraculous, in accord with the reading in 𝓟 (there miraculous was spelled myraculous by scribe 2).
Elsewhere in the original text there are two other occurrences of marvelous power (Mosiah 1:13 and Alma 57:26) and four of miraculous power (Alma 8:32, Alma 49:28, Alma 57:26, and Helaman 4:25). In one case of miraculous power, the text has changed from miraculous power to matchless power (Alma 49:28). Of particular relevance here, we note that in Alma 57:26 both miraculous power and marvelous power occur in the same passage (just like originally in Alma 10:5):
Summary: Restore in Alma 10:5 the original reading miraculous power in place of marvelous power (“I have seen much of his mysteries and his miraculous power”), even though earlier in the verse the text reads “his mysteries and marvelous power”.