Oliver Cowdery initially wrote “I will deliver them in the hands of their enemies”, a highly unlikely reading. Somewhat later, he replaced in with into, probably while proofing 𝓟 against 𝓞 (the level of ink flow is slightly heavier and the insertion itself is wobbly). For evidence that the scribes sometimes miswrote into as in, see the discussion under 1 Nephi 4:33.
The Book of Mormon text has only examples of people being delivered into the hands of others (16 times, excluding Mosiah 11:21); but there are no examples of people being delivered in or unto the hands of others. Thus usage elsewhere systematically supports Oliver’s correction in the printer’s manuscript for Mosiah 11:21.
Summary: Retain in Mosiah 11:21 the corrected reading in 𝓟: “into the hands of their enemies”; Oliver Cowdery’s correction here was probably the result of his later proofing of 𝓟 against 𝓞.