The original manuscript is not extant here. Spacing between extant fragments suggests that yea was probably there, although one could argue that maybe it was missing in 𝓞 and instead there was a subject they before will destroy. In copying to the printer’s manuscript, Oliver Cowdery initially wrote “& will destroy” (that is, without the yea). He then supralinearly inserted the yea with no change in the level of ink flow. There is no grammatical or semantic reason for inserting the yea here, so Oliver was most probably correcting to the reading in 𝓞. Later, in the 1840 edition, the yea was once more lost from the text. It was restored to the RLDS text in 1908; the LDS text has consistently maintained the yea in this passage.
Elsewhere there is definite support for having the transitional yea and followed by a subjectless predicate. I have found seven other examples in the text, including these four where the ellipted subject is a third person plural:
In fact, for the example in Alma 28:12, the yea was omitted, just like here in Alma 57:31 except that the omission occurred in the 1830 edition rather than in the 1840 edition (for discussion, see under Alma 28:12). In any event, there is nothing wrong with having this kind of syntactic construction, where yea and heads a subjectless finite predicate.
Summary: Accept in Alma 57:31 the yea and followed by a subjectless finite predicate, the reading here in the earliest extant sources (𝓟 and the first two editions).