ImHuman provides more humane, fun, easy, secure and mostly user friendly captcha (are you really a human?) detection for posting comments.
Yes, for security measures your plugin requires API user and key values to run. All the API information is public and free.
Yes, you can under "Plugins"->"ImHuman Settings"
ImHuman API returns detailed error codes and error strings in the event of an error. All the information are provided in plain english and easy to understand. If you are still unsure about the error and reasons you are having them, you can visit support pages anytime and seek for help.
Yes, you can. Browse to folder "wp-content/plugins/imhuman/css/" and edit imhuman.css to edit your display style.
With 0.0.8 yes. below 0.0.8 no.
With and above 0.0.8 Yes, JQuery. Below 0.0.8 No.