Hello Rachel. I cannot find a way to ask Dropbox to give me the 1000 most recent files. Dropbox just gives me the first 1000 ePubs it finds and they are sorted afterwards by Marvin. I'm afraid I have no control on it. In the next update you'll be able to selectively pick which Dropbox folder to get books from (although, I don't think this will help in your particular case).
Yes, the next version supports OPDS catalogs with authentication and search support, as well as a built-in web browser designed to download ePubs straight into Marvin. You can also access you Calibre library from it.