The original text manuscript undoubtedly read “from before them”. Oliver Cowdery, when he initially wrote the text into the printer’s manuscript, accidentally skipped the from, but virtually immediately he caught his error and supralinearly added the word (there is no change in the level of ink flow); moreover, the 1830 edition, a firsthand copy of 𝓞, has the from.
But one wonders here if the verb might be flee rather than fall—that is, “they did flee back from before them”. Even if fall is an error, it would have been in 𝓞 since both the 1830 edition and 𝓟 agree here. The verbs flee and fall are consonantally identical, so it is possible that Oliver Cowdery might have misheard Joseph Smith’s dictation. Or Joseph himself could have misread an original flee as fall because of the visual similarity between the two words.
In support of this emendation, there are quite a few occurrences of “flee from before X” elsewhere in the text:
And there is even one occurrence of “flee back from before X”:
Of course, maybe this last passage is an error for “but the king of the Lamanites fell back from before Alma”!
In the King James Bible, there are six instances of “flee from before X”, as in 1 Samuel 31:1: “and the men of Israel fled from before the Philistines”. But there are no biblical examples of “back from before”. In other words, there are no examples in the King James Bible of either “flee back from before X” or “fall back from before X”. In the Book of Mormon, based on the earliest textual sources, there is one example of each type (that is, when back occurs with “from before”, there is one example with the verb flee and one with fall ). The critical text will leave the text as it is in both Alma 2:32 and 3 Nephi 4:12; either reading is possible.
Summary: Keep the unique occurrence of fall back in “did fall back from before them” (3 Nephi 4:12) as well as the unique occurrence of flee back in “fled back from before Alma” (Alma 2:32).