There are two problems here with the current text. First, the present-tense perfect have was inserted in the original manuscript when what we expect is the past-tense perfect had. Whenever hitherto is used in the text, we always get a perfect auxiliary, either a present-tense form of have or the past-tense had. Yet there is a consistent difference that holds everywhere else in the text: on the one hand, the present-tense form shows up when the surrounding verbs refer to present time (15 occurrences), as in Mosiah 2:31: “I would that ye should do as ye hath hitherto done”; on the other hand, the past-tense had shows up when the surrounding verbs refer to past time (9 occurrences), as in Alma 58:1: “for behold they remembered that which we had hitherto done”.
It is therefore quite possible that here in Alma 60:16 Oliver Cowdery accidentally changed the original text by inserting have in 𝓞 rather than had. Even so, when Oliver copied the text from 𝓞 into 𝓟, he started to write “as we had hitherto done”. But shortly after writing the had in 𝓟, Oliver crossed it out and then continued inline with the rest of the text (“hitherto have done”). In other words, he ended up faithfully copying the corrected reading in 𝓞, even though that may have been a mistake for “had hitherto done” (see below for discussion of the placement of have after hitherto).
Despite these considerations, we should note that the present-tense have here in Alma 60:16 may be precisely what Moroni intended to write to Parhoron. Up to the present situation, Moroni is arguing, they have been united in strength. Given this interpretation, have will work. The critical text will maintain in Alma 60:16 the present-tense have despite its uniqueness when compared with other instances of hitherto.
The second problem here in Alma 60:16 has to do with the placement of the inserted perfect auxiliary. Elsewhere in the text, hitherto almost always comes between the perfect auxiliary and the past participle, as in the two examples cited above, in Mosiah 2:31 (“as ye hath hitherto done”) and in Alma 58:1 (“that which we had hitherto done”), and in 21 other cases. The only other exception to the word order is in Alma 51:16: “for behold this had been hitherto a cause of all their destructions”. In that case, the hitherto is placed at the end, after the past participle. The original manuscript is extant for Alma 60:16 and reads with hitherto before the perfect have, so either we have a primitive error in Alma 60:16 or we just have to accept the possibility of some occasional variation in the position of hitherto with respect to its nearby verbs. The critical text will therefore accept the occasional variation in word order for the word hitherto, here in Alma 60:16 (before have done) and in Alma 51:16 (after had been).
Summary: Maintain in Alma 60:16 the earliest phraseology, the corrected reading in both manuscripts, “as we hitherto have done”; the present-tense have will work here because up to that moment the Nephites have been united in their strength; and although the placement of the perfect auxiliary have after hitherto is unexpected, it is possible.