Does this mean Ajenti's running your Node app (as intended) or you used forever/supervisor/etc for that?

Click on the plugin item to expand it and use the quick install button there.

Sorry, what exactly do you mean with "is going to maintain the code"?

Once you have new changes, simply send another pull request.

DNS changes (including DKIM setup) might take up to 36 hours to full propagate.
Make sure to also set up SPF for aid gmail message authentication.

The key size used is the minimum one recommended by Google for GMail sender authentication. If you need a smaller one, please use one of online manuals available.

Simply add platforms=['debian'] to the plugin class. It doesn't have to support all platforms right from the beginning.

Could you please fork our repo & make a pull request? 
Also, please take care of the coding style, I suggest using https://pypi.python.org/pypi/pep8

Reinstalling Ajenti from APT won't reset the config file. You can download the stock config from here: https://github.com/Eugeny/ajenti/blob/master/packaging/files/config.json