In the printer’s manuscript, Oliver Cowdery initially skipped the original “in this”. But when he first corrected his error, he supralinearly inserted “in doing these things”, probably because of the influence of the following clause (“and do all these things”). Later, probably when he was proofing
𝓟 against 𝓞 (which is not extant here), he discovered that his first correction was in error. With heavier ink flow, he crossed out the supralinear “in doing these things”, then supralinearly wrote (again with heavier ink flow) the words “in this” in front of his first supralinear insertion, now crossed out. Most probably, the original text read “there is no harm in this”, the final corrected text in 𝓟. Elsewhere, the text has two other occurrences of the prepositional phase “in this”— namely, “in this I do not glory” (Mosiah 23:11) and “in this ye have sinned” (Helaman 8:24).
Summary: Follow Oliver Cowdery’s final correction of 2 Nephi 28:8 in the printer’s manuscript, “there is no harm in this”.