Yes, ghost supports HTML. With the new editor, you can even take rich content (i.e. content with headings, boldface, italicized, etc.) and paste it in, and the editor will clean it up for you
Yes, you can. There are two options for this - creating a custom export and importing that into Ghost, or using the private (undocumented) write API to insert them into an existing Ghost instance
Ghost saves posts in the mobiledoc format, and internally converts that mobiledoc to html. Here’s the template to use:
The best way to figure out how to use the private API is reading the private API information section in the API docs (so you get a better understanding of the core concept), and inspecting network traffic (using your browser’s developer tools) as you go through the actions of creating a test post