Simplifying, Textpack is a markdown file, plus optional image files, packed into single zip file.
Yes. The Trial version is limited to 20 indexed tasks and 20 indexed documents.
You will find a short guide how to configure workspace synchronization in the attached video.
No. Currently Keepmark is a desktop application. However, documents are stored in a user-accessible form and can be synchronized with mobile devices, e.g., using services like Drobox or the WebDav protocol.