The printer’s manuscript has “unto the people”, but the 1830 edition reads “unto this people”. Elsewhere the text has 18 examples of “preach (un)to the people” and 6 of “preach (un)to this people”. (Here I exclude cases where people is postmodified since we expect only the in such cases.) So both readings in Mormon 1:16 are theoretically possible.
Under Helaman 14:20, I list four independent cases where scribe 2 of 𝓟 replaced this with the. On the other hand, there are no clear cases where the 1830 compositor set this instead of the (there is one case, in Mosiah 1:10, where he set the instead of this). These transmission errors argue that the 1830 reading in Mormon 1:16, “unto this people”, is probably the correct one.
Summary: Retain “unto this people” in Mormon 1:16 (the 1830 reading) rather than “unto the people” (the reading in 𝓟); we have specific evidence that scribe 2 of 𝓟 sometimes accidentally replaced this with the.