1. Depends if they were overwritten. If not, you can usually recover them just fine (eg. photorec)
2. Depends on format, but most likely no, unless explicitly enabled (iirc in Word when you have multiple authors you can track changes. But not on by default)
3. If someone has access to older file, sure. There is also a lot of metadata in most document formats - including version of the editor, date, format, operating system version, username and whatever else computer was able to get.