I think I have a similar issue, that I am trying to fix.
I have managed to have the list item that I want to edit as a hidden attribute in my form, so that when the user accesses the form next and any data that they enter is updated to the form.
The problem that I can't overcome, which i think is the same problem you are having, is that I don't know how to reference to the item in my list based on the ID in the form.
I am using the following commands to add data to the list
batch.SelectSingleNode(
"/Batch/Method/Field[@Name='Seq2ActivityClosedDate']", NamespaceManager).SetValue(seq2ActivityClosedDate);
but all itll do is add a new entry into the list, where as I want to add this data to the existing list/xml document.
Were you able to achieve this?