The 1830 typesetter accidentally misread the word mediator in 𝓟 as mediation. This incorrect reading was perpetuated in the LDS text until the 1981 edition, but the word mediation continues in the RLDS text. The following verse repeats the word mediator and implies that the 1830 typesetter’s change in verse 27 was not due to editing but instead to his misreading of the copy-text:
In the earliest textual sources (in this case, the printer’s manuscript), both verses refer to “the great Mediator”. The word mediation occurs nowhere in the text, while the word mediator occurs nowhere else except here in 2 Nephi 2:27–28. However, mediator does occur fairly frequently in the King James Bible (seven times in the New Testament epistles), but mediation is not found at all in the biblical text.
Summary: Maintain both occurrences of the original reading “the great Mediator” in 2 Nephi 2:27–28; the 1830 change of the first mediator to mediation is apparently the result of the 1830 compositor misreading his copy.