The 1830 typesetter accidentally, it would appear, switched the order of be all to all be here in Mosiah 25:20. Elsewhere in the original text, the order is always all be:
However, in one of these cases, the order has been switched to be all in the LDS text:
The three other cases of “all be fulfilled” have maintained their original word order. The word order be all is clearly possible; note that be all was introduced into the 1879 LDS edition in Alma 34:13 and has been retained in the LDS text ever since. The critical text will in each case follow the word order as found in the earliest textual sources, either all be or be all. This means that there is a unique occurrence of the order be all here in Mosiah 25:20, although the possibility remains that this unique word order could be an error.
Summary: Restore in Mosiah 25:20 the unique word order be all in “they could not be all governed by one teacher”; the dominant order all be will be restored in Alma 34:13 (“it shall all be fulfilled”).