The 1874 RLDS edition introduced the negative not here, giving “I do not joy in receiving your epistle”. The original manuscript is not extant at all for this part of the text, but nonetheless there is really no need to inject a not here. Perhaps the typesetter (or the editors) for the 1874 edition were influenced by Parhoron’s initial statement at the beginning of his epistle: “I do not joy in your great afflictions” (Alma 61:2). Or perhaps the unusualness in modern English of do joy led someone to think that a not was missing here. (For another example of incorrectly inserting a not after the auxiliary verb do, see under Mosiah 26:7–9.)
At the end of his epistle, Parhoron admits that he was relieved to have gotten a letter from Moroni since he “was somewhat worried” concerning what should be done against the rebels; in fact, Parhoron wasn’t even sure “whether it should be just in us to go against our brethren” (Alma 61:19). Moroni made sure that this indecision on Parhoron’s part did not continue; it seems here that we have somewhat of a confession from Parhoron that he had not been that active in opposing the rebels; in other words, Moroni had been justified in threatening Parhoron. Although it is possible that Parhoron did not “joy in Moroni’s epistle”, he definitely did “joy in receiving Moroni’s epistle”.
Summary: Maintain the lack of not in Alma 61:19; Parhoron, in his positive reply to Moroni, wanted to emphasize that he was happy to receive a communication from Moroni.