1. The app can just be an html, css, and js file with the exception of the splashscreen plugin (and the statusbar plugin if you want it to scale well on iPad). Also, you will need graphics for the icons and splash screens and of course a config.xml file with the configuration for name and id, icons/splashes and a couple of general preferences
See the Phonegap Build Docs
2. You don't implement 'dimensions' for all different devices. Similar to webdesign (where you really don't know at what size the user opens and scales the browser) you simply create something in 'liquid design' which is supposed to scale well with the various screen dimensions and resolutions.
Also, make sure you look up a bit about the html meta element for 'viewport'
3. You can build in Windows, iOS or Linux (or any other OS) when using Phonegap Build. However, iOS apps can only be published if you have a Mac (be it physical or in the cloud)
4. The benefits are that you can have multiple apps simultaneously in your account, and the max size of each is larger than in a free account. The benefits for Adobe are obvious: they get your money.
Thank you for your fast response.
It will take me some more time to go through the information, you provided.