Thank you for contacting Just Answer.
The problem with digital files is that there really is no way to "return" them. For example, if the former editor were to email the manuscript back to The Write Elements, a copy of the file would still remain on his computer. The only thing that can be done is for The Write Elements to ask the former editor to delete the file from his computer, and even then there would be little way for The Write Elements to verify that he had complied. Depending on the confidentiality agreement that he signed with The Write Elements, he may or may not have an obligation to delete the file or return his hardcopy of your work. It might be a good idea to ask The Write Elements to let you look at the confidentiality agreement; they would have no obligation to let you see it however.
As far as copyright infringement goes, no infringement takes place until someone makes a copy of your work (or a portion thereof) without your permission. By keeping a hardcopy of your manuscript or refusing to delete his electronic copy, the former editor has not committed copyright infringement. You are correct: you would have to wait until an unauthorized copy of the work shows up for there to be a violation. Frankly, the fact that someone who worked on your manuscript at The Write Stuff has quit makes copyright infringement neither more likely or less likely: it is just as possible for a current employee to commit copyright infringement as it is for a former employee to do so.
Remember that your work was protected by copyright from the moment that you wrote it: you do not have to wait until the copyright has been registered with the Copyright Office for it to be protected. If, presumably, your intent is to publish the work, you will always face the possibility that some nameless person is going to copy it. The fact that one person whose name you don't know saw your work before it was published doesn't greatly increase the likelihood that there will be a copyright infringement of your work.
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My answer is not legal advice. This forum is only designed to give you a basis of knowledge. I am not your attorney. You should consult with legal counsel in your state for specific information relevant to your situation. Please remember to hit "ACCEPT"!