Simplifying, Textpack is a markdown file, plus optional image files, packed into single zip file.
The two-key mechanism allows you to cancel the purchase in case of a refund request.
See attached video to see how to configure email account in Keepmark.
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.
There is no limit for number of devices to run Keepmark for a single license.