Here Oliver Cowdery initially wrote “unto your hands” in 𝓟; then somewhat later (with heavier and uneven ink flow) he corrected the unto to into. His correction probably occurred when he proofed 𝓟 against 𝓞. The text has many examples of “to fall into one’s hands” (27 times, including here in Mosiah 17:9) but none of “to fall unto one’s hands”.
The tendency to mix up into and unto is fairly frequent in the manuscripts. For a list of ten cases where Oliver Cowdery initially wrote unto instead of the correct into, see under 2 Nephi 8:23.
Summary: Maintain in Mosiah 17:9 the corrected reading “I have fallen into your hands”; the text consistently supports the use of into rather than unto in the expression “to fall into one’s hands”.