Oliver Cowdery sometimes added the auxiliary verb do in the manuscripts, if only momentarily. Here in the printer’s manuscript, he initially wrote “which did believe”. Later he crossed out the did and inserted inline a d at the end of believe. These corrections are with slightly heavier ink flow and were probably made when Oliver proofed 𝓟 against 𝓞 (no longer extant here). There are a few other cases in the text where Oliver added the do auxiliary, usually in the context of other instances of the do auxiliary. In two cases, the do auxiliary was permanently added to the text when Oliver copied from 𝓞 into 𝓟:
(See under those two passages for discussion.) In a couple of other cases, Oliver initially wrote the do auxiliary in 𝓟, perhaps in the second case because of a nearby use of the do auxiliary (marked below with an arrow):
In the first case, Oliver apparently started to write did bear in 𝓟 (for discussion, see under Alma 53:13). In the second case, he started to write did thrust in 𝓟. Here in Helaman 9:39, Oliver’s initial did believe was probably influenced by the use of the auxiliary form did earlier in verse 37: “for according to the words he did deny and also according to the words he did confess”. On the other hand, the intervening verse 38 has only instances of the be verb (“and he was brought to prove that he himself was the very murderer insomuch that the five were set at liberty and also was Nephi”); nevertheless, these verb forms would not have interfered much with the influence of the preceding instances of did in verse 37.
There are 12 other occurrences in the earliest text of “ believed”, including one more in Helaman 9:39: “and there were some also which believed because of the testimony of the five”. But there are also two examples in the earliest text of “ did believe”:
These two passages parallel the first one in Helaman 9:39 (all three refer to believing in or on someone’s words). Thus either reading is theoretically possible in Helaman 9:39, and we can assume that Oliver Cowdery’s replacement of did believe with believed in 𝓟 was not influenced by grammatical considerations, only by his attempt to correctly copy the text from 𝓞 into 𝓟.
Summary: Accept in Helaman 9:39 the corrected reading in 𝓟 of believed instead of did believe;although not extant here, 𝓞 probably read believed since either believed or did believe is possible.