Don't try to set any nme values. The nme Object is given to you as a parameter in the nh_NativeMouseClicked event sub. It contains all the info about the mouse input event (like x,y values, mouse button and click count). You'll have to run the code to see which mouse button is represented by which integer, it's been a while since I tested it. But it should be the same across platforms.
As for unregistering, yes, you definitely want to unregister the NativeHook before your program closes. If you have too many registered NativeHooks, you'll have difficulty registering new ones and other programs that rely on system input events will suffer interference. If this happens, just reboot your computer and it will clear the dangling NativeHooks. However if you properly unregister your NativeHooks when you're done with them, you won't have this problem. The jAWTRobot library has a JVMShutdownHook function that allows you to run some code as the JVM is shutting down under its own control so you can perform clean up tasks. This is a good place to unregister NativeHooks in non-UI apps. In UI apps, you can use the MainForm_Closed event or something.
nh.endNativeMouseInputListener 'This stops the NativeMouseInputListener. This is NOT the same as unregistering the NativeHook
nh.unregisterNativeHook 'unregisters the NativeHook
All these functions should appear in the IDE's code completion menu along with copious comments explaining their usage.