Hello all :)
is there a way to restore a previous version of a document (from libary) with nintex workflow for SharePoint 2013? (eg after a rejected content approval)?
Depending on your process, then yes you could. Just have to ensure your order or workflow actions do what you need to achieve. You have "delete previous versions" and "delete drafts" actions that can help you achieve this. You could also force check out, then have any edits made and if approved, check it back in which would commit to a new version, otherwise discard check out which essentially deletes any edits.
Lots to be considered here but depend on what you are really aiming to do.