How about if Save & Continue is only possible after the first submit? Then, the form's name is updated, they can re-open from the library and continue on their way? Or, perhaps you check for that file name as the first step on your "Save & Continue" - skip it for new forms that haven't been submitted, but for forms that have, you could get the name right before the second submit - that would give the workflow a chance to run, right?
Hilary Stoupa
