I am currently reviewing the Nintex Products. A question I have would be is it possible to edit and maintain Nintex developed forms and workflows with InfoPath and SharePoint Designer?
If i understand your question correctly you want to maintain i.e. edit Nintex forms using Infopath? Then the answer may be "No" as you need Nintex designer which would be licensed (development part of your support assurance). Nintex can run without any issue but changing anything you may need designer (if that makes sense ?)
I second what the others have said. I will add, though, that Nintex Forms and Workflows are just XML so, if you truly wanted to, you would be able to modify them using any text or code editor. It would be an absolute nightmare, and I strongly suggest against doing so, but it is possible.
The best solution is to simply work with them in the browser, as intended by the application.