Ghost headless just means you don’t have to use the Ghost-generated frontend for your site. You use the Ghost backend to pull data in to your frontend, whatever that may be
This is a really broad question, so it’s kind of difficult to answer
Yes, Ghost needs to be hosted on a server to work. The Ghost Docs instance is hosted on Ghost Pro
Ghost can be run in a subdirectory, but it cannot be hosted on the /ghost subdirectory. Similar to 1., this question needs more clarity.
To be clear, when it comes to saying Ghost can be headless, it basically means there is a robust API available for you to interact with your instance
Well, NetlifyCMS works in a kinda different way where we have to just add two files at the root of the directory admin and inside that admin directory we have index.html and config.yml.
Index.html contains that core of the netlifycms which render the netlifycms interface and config.yml is to configure, how we want the posts to show up in the CMS.
Well, it is good idea to add similar concept the to Ghost and I really like the Ghost admin interface.