Usually the earliest textual sources have the standard past participle fallen. But here in Mosiah 4:1, the earliest text has fell (“they had fell”). The use of the simple past-tense form for the past participle occurs fairly often in the earliest text (for a general discussion, see under past participle in volume 3). Besides here in Mosiah 4:1, we have the following instances of past participial fell:
Except for the last instance, Joseph Smith replaced all of these cases of fell with fallen in his editing for the 1837 edition. The last one was changed (in the LDS text) in the third printing of the 1905 LDS missionary edition (in 1907). The RLDS text made the change in the 1953 RLDS edition. There is one case in the original manuscript (Alma 56:51) where Oliver Cowdery initially wrote fell (spelled as fel ) as the very last word on a manuscript page (at the end of line 35 on page 346ªof 𝓞); then he crossed out the fel and wrote the correct fallen as the first word on the next manuscript page (at the beginning of line 1 on page 347ªof 𝓞). It is worth noting, however, that the e of fel is not fully legible and thus the final word may have simply been fal—that is, Oliver started to write fallen but found that he didn’t have enough room to write the whole word, so he crossed out the f al that he had written and wrote the entire fallen on the next manuscript page. In any event, there are five cases where the earliest textual sources support the perfect form “have fell”. In 63 cases (including the case of Alma 56:51) the earliest textual sources support “have fallen”. There is no variation for the past participle fallen when the helping verb is the passive be or the archaic perfective be (for verbs involving change), or when we have an adjectival past participle. Although the five cases of perfective “have fell” could be due to dialectal overlay, the critical text will accept them as original.
Summary: Maintain the five cases of original “have fell”, including here in Mosiah 4:1 (“they had fell to the earth”).