No, Ajenti has become free for hosters a while ago (see: http://ajenti.org/licensing). If you have obtained the information about licensing on a third party website, please let me know and I'll ask them to update it.

I understand it that you want to control a remote BIND server from Ajenti. It's not straightforward, but possible. You need to use NFS to mount the folder containing BIND configuration onto the Ajenti machine, and create a service initscript for BIND which would use SSH instead of starting BIND locally.

Да, разумеется. Любое использование кроме встраивания в коммерческий продукт бесплатно: http://ajenti.org/licensing

That's typically caused by incorrect arch of packages with native extensions, i.e. python-gevent and python-greenlet

Hi, you when your dialog is done (i.e., fires 'button' event), you need to .update() the binder, so that your data objects are updated with the new data from UI. Example: https://github.com/Eugeny/ajenti/blob/master/ajenti/plugins/configurator/configurator.py#L228-L248

Only a guide on installing V: http://support.ajenti.org/list/27624-kb/?category=9332
There are guides on setting up websites, those might help you too

It's pretty straightforward. Just set up a website and add a "reserse proxy" type content entry.

Yes, you can set up reverse proxy vhosts with Ajenti V.

You don't need SSL between NGINX and Ajenti. Disable SSL in Ajenti and configure it in NGINX instead.

Ajenti V mail doesn't include any webmail app. But you can instal one of your choice through Ajenti V.