To get steam overlay working:
1. Open the exported nw folder of your game.
2. Rename package.nw to package.zip and open it with winzip or winrar.
3. In the zip, find and open package.json with notepad and add "--in-process-GPU" as the last arg in the code.
4. Save and close
5. Rename the package.zip back to package.nw
6. Upload or open with steam and overlay should work when clicking shift+tab.
And read this thread and you'll find solutions for most of your steam problems.
You can also change so NW exports the game with the in-process-GPU arg so you don't have to redo this every time.
Good luck with your project!