I am building a Nintex workflow for a SharePoint 2013 list. I am making continuous changes, and I quickly run up the number of versions. Is there a way to (a) start an unpublished workflow as I am the only one who needs to run it, or (b) to unpublish a workflow so I have version 1.1, 1.2, 1.3, whatever and then publish it as 2.0 when it is ready to show to someone else?
No, this isn't possible. The entire point of publishing a workflow is to make it available to run.
What you could do is publish it under another name for as long as you are testing. Once you confirm it's working as you intended, publish it under the initial name and overwrite the 'public' version.
Concerning the version numbers, every time you save the workflow a minor version is created (the number behind the dot). Everytime you publish it, a major version is created (the number before the dot)
If you want to revert to an older version, you can do this by opening an older version. On the ribbon, click the open button and tick the checkbox to display previous versions.