Depending on the maximum aperture of your lens, when using a 2X teleconverter your max aperture is multiplied by 2 so a F 5.6 max aperture lens becomes an F 11 lens and any working aperture smaller than F8 is likely going to be a struggle for the AF system of any camera that were designed to work up to about F 8. Anything smaller and most of the time the body won't even find focus and at those aperture settings, the viewfinder gets so dark it will always be a struggle to find correct focus.
What does "REALLY HARD" mean here? Does that mean the focusing ring PHYSICALLY is difficult to turn? Or do you mean the image in the viewfinder does not seem sharp? Or do you mean something completely different?
Hi, the lever in the lens controls the aperture of the lens and is sprung loaded so the iris closes when not activated - (to help stop dust getting onto the more vital elements of the lens, especially when changing lenses - I think)
The extender has a similar lever to transfer the setting from the camera to the lens, but does not need to be sprung loaded (it only has fixed magnifying lens and no moving elements), so it feels unfastened but that is how it's designed and is not a problem.
Someone else can give you better knowledge about autofocus and long lenses - I think it's something to do with lower contrast which some AF systems use as their means of telling if the image is in focus.