1.)I use an Xbox 360 wired controller it works great.
2.)Not sure, I use a controller because I can't move/aim with mouse and keyboard, I haven't encountered any problems so far with being able to play effectively.
3.) Yes, for some things. If you choose a controller as your input device then it will be your main device for interacting with the environment and NPC's. You can use the mouse for your menu items and manipulating the map as well as using skills on your hotbar. I usually use a mouse for crafting as I'm stationary and don't move around.
4.) Not sure.
5.) Not sure.
6.) There are good threads about this here, one I liked was a guideline for controller hotbar layouts. Set up a crossbar that is shared between all classes that will have commonly used functions like sprint, teleport, mount/dismount, FC, Journal etc. Disable hotbars you don't need to cut down on clutter. Use macros too, the downside to the controller is the limited space.