chs. 1-7, chs.8-9, and chs. 10-13) and how to adequately deal with the seemingly incongruous parts.
Here is a great quote in summarizing the difficulties from James Dunn in his latest, Christianity in the Making: Beginning From Jerusalem:
For myself, such hypotheses have the advantage of making sense of the puzzling factors...My only problem is with envisaging the situation and the motivation which caused some anonymous collector or editor to chop off the introductions and conclusions to each letter and simply stick the torsos together in such an awkward way as to raise the questions which the various amalgamation hypotheses are designed to resolve. Why not retain them as complete letters? Nothing was obviously to be gained by giving the impression that Paul wrote only two letters to the Corinthians rather than, say, five or more. If the editor felt so free to 'top and tail' the letters in question, what prevented him from exercising the same freedom to edit the material into a more coherent unit? Or if he was careful to excise greetings, thanksgivings and farewell, would we not have expected him to take care to ensure better links between the sections? Furthermore, unless the editing was done very early indeed, then we might have expected copies of one or more of these independent letters to have been made and circulated more widely, which would almost certainly have left some mark in the textual tradition. But of that there is none.
I do not believe the puzzle of 2 Corinthians is finally resolvable. The unavoidable fact is that all the data of the letter in its present form are capable of supporting a variety of hypotheses. What is frustrating in this, as in other debates on the beginnings of Christianity, is the unwillingness of some to make allowances for changes of circumstance or information or mood which might provide a perfectly adequate explanation of the various infelicities and disjunctures which grate on the ear of the twentieth-or twenty-first-century reader of such documents. The inadequacy of our historical imagination is often a greater problem than the puzzling data of a letter like 2 Corinthians (emphasis mine; 835-836).
Having been an advocate of one form of the various partition theories, I have since revised my thinking. To me the clincher is not having any evidence for any of these partition theories in our manuscript evidence. Some might accuse this as being an argument from silence, but I steadfastly maintain that a cautious and careful interpreter of this letter will ultimately be swayed by this inevitable fact. Furthermore, rhetorical conventions cannot and should not be set aside when evaluating these letters. Dunn is spot on when he accuses the historian for the lack of imagination in making allowances in viewing the text as an organic whole.