I have written few custom checkstyle rules using checkstyle API. They run fine using Maven (after I add the new project as a dependency to the checkstyle plugin).
No
You already have the correct links that will eventually get you there. As for your questions:
All your custom checks can go into one JAR file. That JAR file must be an Eclipse plugin JAR. I simply install it by copying it to the Eclipse dropins folder, but there may be more elegant ways to do that.
So you end up with two plugins: The original, unmodified Eclipse-CS, and your own plugin which contains the custom checks. When both are independently installed in Eclipse, the Eclipse-CS configuration dialog will offer your custom checks for use in Checkstyle configurations.
You can do it like following :
You can refer this tutorial