My main concern with the 6D is ending up being frustrated by the outer points AF performance.
If this is your main concern, you *will* get frustrated, the outer points aren't fully capable points but rather tracking helpers and a convenience for very good light conditions. With the 6D, you will have to focus & recompose, if that's not what you want then the 6D isn't for you.
I'd also rather have a 5d3, but it's nearly double the price ... fortunately I was well informed before I made the decision that the 6D is essentially a one focus point camera - and now you also know.
Can you configure the camera to select the AF point directly through the multi controller (the inner part of the rear dial), without having to press first the dedicated AF area or point button?
You can either set the af points with the multicontroller, or press the top right button and then use both wheels. Both options aren't very quick to navigate, btw.
And how do you set the camera for an instant return to the center AF point? By pressing the SET button?
No, this is not possible with the Canon firmware - and when they removed this very obvious option from SET they were probably rolling on the floor, laughing and sniggering "Just get a 5d3, you jerks!".
But this will be fixed by Magic Lantern, so SET will have a as "set af to center point" even if Canon doesn't want us to have it.