Here Oliver Cowdery initially wrote the plural idolatries in the original manuscript (probably because all the preceding nouns are in the plural). He then erased the ies at the end of the word and overwrote the erasure with a large y and an ampersand. The ampersand was smeared, so he overwrote the ampersand. Even so, when Oliver copied this passage into the printer’s manuscript, he omitted the and; perhaps the smearing made him think the ampersand had been crossed out. In the previous verse, Oliver made virtually the same error in his copywork, thinking that an ampersand in 𝓞 had been crossed out in 𝓞 because of smearing from an immediately preceding erasure, thus omitting the and in 𝓟 (see the preceding discussion under Alma 50:19–20).
Here in verse 21, supplying the and after idolatry improves the parallelism of the first three pairs of conjuncts: “their quarrelings and their contentions … their murderings and their plunderings … their idolatry and their whoredoms”.
Summary: Restore in Alma 50:21 the and that Oliver Cowdery accidentally omitted between their idolatry and their whoredoms when he copied the text from 𝓞 into 𝓟.