This correction in 𝓟 of unto to into appears to be an immediate one and reflects an apparent restoration of the reading of the original manuscript (which is no longer extant here); the level of ink flow is unchanged.
The use of into in the context of the phrase “everlasting fire” is supported in one other place in the text:
In the printer’s manuscript for Mosiah 26:27, the unknown scribe 2 initially wrote unto, then erased the u and overwrote it with i. Erasure is a clear indication of an immediate correction and is strong evidence that the original manuscript read into in Mosiah 26:27. This same phraseology is found in Matthew 25:41 (“depart from me ye cursed into everlasting fire prepared for the devil and his angels”).
Summary: Retain into, the immediately corrected reading of the printer’s manuscript for 2 Nephi 9:16.