I think you need to make a separate Entity with the other attributes and reference to this FileDocument entity. That way you can safely delete the files without removing the object with the other attributes.
As far as I know, there is no other widget in the AppStore that only supports uploading one file. You could see if you can do a feature request for the DropZone widget on Github, asking for a single file upload, instead of multiple
I think, data grid extension will help you. In this add-on, we will be having inline buttons option. Using this, provide a action button in a column, call a microflow on click of this button, delete file object of this particular entity and commit it. Hope so, it may work.