I think there's been a bit of overanalysis on this question. Let's say that we simplify thesentence:
Emily Dickinson’s letters to Susan Huntington Dickinson were written over a period of twenty years, outnumbering her letters to anyone else.
If this were the original sentence, then it would be just fine IMO. It isnot true that a concluding participial phrase must modify the entire preceding clause--it can also modify anindividual noun in the clause instead. I think the main problem with
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Emily Dickinson’s letters to Susan Huntington Dickinson were written over a period of twenty years, outnumbering her letters to anyone else.
If this were the original sentence, then it would be just fine IMO. It isnot true that a concluding participial phrase must modify the entire preceding clause--it can also modify anindividual noun in the clause instead. I think the main problem with
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