Hi,
Here are the answers to your questions:
1. WTP automatically takes screenshots when a test case fails. It can also be triggered programmatically during a test.
2. WTP supports standard logging functionality.
3. Headless mode is NOT supported.
4. WTP can be used with Ant or Maven in Jenkins or other CI environments.
5. It includes basic documentation and JavaDoc. Online documentation has unfortunately been deleted by Google (I will try to get it back online on Github).
WTP does not cost any money, but if you want to use it's full potential, you'll need to invest some time and effort to adapt your testing environment to it.
In special cases you might need to adapt or extend WTP to fit your needs.
HTH,
Fred