Here in the original manuscript, Oliver Cowdery initially wrote “for it were not all fulfilled”. Virtually immediately he supralinearly inserted was (there is no difference in the level of ink flow for the was); then he seems to have redipped his quill just before crossing out the were and writing the insert mark (both the crossout and the insert mark were written with heavier ink flow).
Normally, the text prefers “for it was”, with 17 occurrences elsewhere. On the other hand, “for it were” does occur twice; and in each case, were is immediately followed by not (just like here in Alma 25:15):
Nonetheless, these two instances read “for it were not possible that … ”; the reference to possibility allows for the subjunctive were, while in Alma 25:15 the indicative is appropriate for the factual “for it was not all fulfilled”. In any case, the use of the correcting was seems to be fully intended, especially since it appears to be a virtually immediate correction. The critical text will follow the corrected text here in 𝓞, “for it was not all fulfilled”.
Summary: Accept in Alma 25:15 the corrected reading in 𝓞 (“for it was not all fulfilled”); in this instance, Oliver Cowdery replaced his initial were with was.