If a student submits an assignment, they can go back to that submission by re-opening the assignment. They will see the current submission as well as a link to resubmit (as long as the assignment isn't closed).
Not that I know of. When I have students doing longer assignments, they write in a Google Doc and submit checkpoints as a URL rather than using the RTE. I can see their progress and they don't lose any work. Those checkpoints don't necessarily have to be separate assignments, either. They can continue to resubmit the same one.