Well, I need to paste a lot of unreleased docs to explain all things clearly, so I would suggest to wait a few more days for the doc to be online.
Some short answers here,
1. key should be used with the right version, and the right edition.
2. EasyAR SDK 1.0 is free, no limitations.
3. EasyAR SDK 2.0 Basic is free, no limitations.
4. EasyAR SDK 2.0 Basic is the upgraded version of EasyAR SDK 1.0, with a lot of bug fixes, suggest to use this.