A short while ago we had two people with the same problem of not being able to check in documents. One had a Pop-Up blocker set up on their machine. It stopped them from downloading documents and stopped them for checking the files back in. Once the pop-up blocker was set to allow pop-ups from the docuware server they were also able to check in their documents.
The other person was using Microsoft Edge as their browser. We had them switch to Google Chrome and they were able to check in their documents. I don't know why Edge had a problem with checking in documents. We had a help ticket open and switching the browser was the recommendation of the person we were talking to.
I do not know of any way to download the original without going into the document history, so I cannot help you with that one.
Hope this helps.