The editors for the 1920 LDS edition added a second instance of the adverb back in this passage, apparently because they wanted to make sure that readers would understand that this use of “from time X until event Y” went backwards in time. Earlier this verse explicitly uses the adverb back for an initial instance of “from time X to event Y” (namely, “from the time that they were destroyed back to the building of the great tower”). One could argue that with already one instance of back, the text does not need to repeat the back later on in the same passage (the use of “yea and even” would imply as much). On the other hand, it is also possible that the original text actually had a second back in this passage and that it was somehow lost during the early transmission of the text. Yet there is no evidence elsewhere in the text for the loss or addition of the word back.
There is one other place in the text where we have a case of reversed sequencing of events, and in that instance there is no back either, not even earlier in the passage:
The text here refers to all the holy prophets from Moses to Abraham. Yet Abraham preceded Moses, just as the day of creation preceded the tower of Babel. Helaman 8:16 thus provides an independent example of reversed sequencing of events—and without any preceding use of back to help the reader. Instead, the reader is simply required to figure out the sequencing from general knowledge. Note further that in Helaman 8:16 the 1920 LDS edition did not insert a back (which would have given “from his day even back to the days of Abraham”). The probable reason for not inserting the word back in Helaman 8:16 was because the editors were not prompted to do so by any preceding occurrence of back in that passage. In Mosiah 28:17, on the other hand, the use of back earlier in the verse provided the impetus for making the emendation.
The critical text will therefore reject the extra back that was inserted in Mosiah 28:17; the example from Helaman 8:16 shows that readers are expected to figure out whether the sequence of events is reversed or not. The general case in the text is that the sequencing of events goes forward in time, and in most of these instances the adverb down is used, as in the following sampling:
Notice that the example in Ether 1:5 takes the opposite direction of what we have in Mosiah 28:17. In some sense, back and down are being used contrastively in these expressions involving sequencing of events. Even so, there are cases of forward sequencing where down is not used, as in one more example that deals with the tower of Babel:
(For another example of forward sequencing without down, see 3 Nephi 26:3.) Thus there are a few cases where neither back nor down are used; in such instances, the reader must figure out the direction of the sequencing. The critical text will therefore follow the earliest reading for each case of forward or reversed sequencing; back and down will not be added to those cases where the text does not explicitly state the direction of the sequencing.
Summary: Remove in Mosiah 28:17 the adverb back that the 1920 edition added to the LDS text; as in Helaman 8:13, back is not needed to specify that this passage is a case of reverse sequencing; similarly, down can be used to specify forward sequencing, but it is not necessary (as in Ether 1:3).