The editors for the 1920 LDS edition shifted the position of the conjunction and in order to eliminate the difficulty of parsing what could be interpreted as two improperly connected participial clauses at the end of this sentence. There may indeed be some primitive error in the printer’s manuscript (the earliest extant reading here), but there is also evidence that the earliest reading could be the original reading.
Note first of all that the 1920 editing for this verse creates an oddity in that the resulting text refers simply to “suffering much”. There are 11 other occurrences in the text of “suffering much”, but in each case there is always some additional information as to what the suffering is. Sometimes the word much is adverbial, other times adjectival (in which case there is a following noun):
The earliest reading at the end of Mosiah 28:4 seems to mean that ‘they suffered much fear that they should be cast off forever’. Of course, the text has the gerundive form fearing rather than the basic noun form fear. However, there are examples in the text where a gerundive noun does follow much:
Thus one way to interpret fearing in Mosiah 28:4 is as a gerundive noun (meaning ‘fear’) rather than as a present participle.
The use of the and before “suffering much” separates off the participial clause, yet the and does give the sense of ‘also’. The original text here is not that difficult to process and has been kept unchanged in the RLDS editions. It should also be noted that the original text had several examples of this same kind of separated present participial clause at the end of the sentence, as in the following nearby examples:
Notice, in particular, how the example in Mosiah 23:13–14 first uses the verb trust as a finite verb (“that ye should trust no man”), then follows it with the present participial trusting (“and also trusting no one”). Similarly, here in Mosiah 28:4, we first have the finite verb suffered (“they suffered much anguish”), which is then followed by the present participial suffering (“and suffering much fearing”). For other examples of this kind of separated present participial usage, see the discussion under Mosiah 23:13–14.
Summary: Restore the earliest reading in Mosiah 28:4, with the and before suffering but not before fearing; the word fearing should be interpreted as a gerundive noun meaning ‘fear’, not as the head of a present participial clause.