In the past, I've had problems with Struts not dealing with overloaded methods properly in ActionForm beans. I'm not sure if the bug has been fixed or not, but I always make it a practice to name the indexed getter and setter something different than the non-indexed ones.
In your example, I'd have a getSubmissions and setSubmissions method to deal with the entire collection, and a getSubmission and setSubmission method to deal with a single indexed entity.