I like @flow’ solution. This is probably the best solution if you only want to store the latest information of who edited which paragraph.
Each paragraph has a unique ID that you could access using ytype._item.id. You could use that unique ID instead of the GUID that you are generating for each div. Furthermore, you could maintain a separate map that maps the unique id’s of the paragraphs to the user that last-edited the paragraph. This would also allow you to assign additional information to the last-edited paragraph.