Verse 15 here is a sentence fragment since no predicate is assigned to the subject noun Teancum. After a long excursus dealing with Moroni rather than Teancum, verse 16 just starts over and ends up describing the orders Moroni sent Teancum (“and it came to pass that Teancum had received orders to …”). The 1920 LDS edition attempted to fix this passage, at least in part, by inserting an and before had begun (originally had began), which definitely makes the fragmented sentence itself read better (“who had established armies to protect the south and the west borders of the land and had began his march towards the land of Bountiful …”). It is quite possible that Oliver Cowdery omitted an and here in 𝓞 since such an error is fairly common for him in the manuscripts (see the examples discussed jointly under 1 Nephi 17:39–40 and 1 Nephi 17:40).
Another possible emendation would be to replace the first had (the one immediately after who) with having (although this change still does not correct the fragmented sentence):
But there are no examples of mix-ups between had and having in the manuscripts, although there has been some editing in the printed editions (in the 1837 and the 1920 LDS editions) where these two verb forms have been intentionally switched: namely, in Mosiah 10:7, Mosiah 23:1, Mosiah 29:42, Alma 16:21, Alma 19:17, Alma 56:10, and Mormon 1:7. But in the manuscripts, it seems more likely an and was omitted in Alma 52:15 than the first had was an error for having. Since the earliest reading for the fragmented sentence does seem to be especially difficult, the critical text will accept the 1920 emendation that supplies an and between the two finite predicates headed by had.
As far as the fragmented sentence itself is concerned, Mormon was apparently unable to recover from his initial switch in topic from Teancum to Moroni. And we can find evidence for this kind of sentential incompleteness elsewhere in the text. In the following example, king Limhi (the speaker here) specifies a subject, in this case Zeniff, who is then referred to by means of a series of relative clauses and present participial clauses, yet Limhi never provides a finite predicate for the initial noun Zeniff or its pronoun he. In fact, Limhi also switches the topic in this passage, from Zeniff to king Laman:
Here in Mosiah 7:21–22, Limhi ultimately ends up starting over with a summarizing statement (“and all this he done for the sole purpose of …”). The critical text will maintain the original fragmented sentence in Alma 52:15–16 as well as in Mosiah 7:21–22.
Summary: Accept in Alma 52:15 the and that the 1920 LDS edition placed between the two finite predicates in the relative clause (thus “who had established armies … and had began his march”); the original fragmented sentence here will be maintained since such incomplete sentences can be found elsewhere in the text.