Hello,
- no idea, sorry
- I wish we could pick focus area directly in the VFB
For distance, don’t forget you can use an object to set the focus distance in the new Corona Camera. This can either be an object in the scene, or a dummy object - the target object can be animated too so that the focus moves during an animation. In terms of strength of the DOF, that still comes down to the aperture etc, just as in the real world, but the focus target object is very handy for setting the distance itself 
On the arc of white, not sure of the cause, one question comes to mind - does it change with the DOF? If you strengthen / weaken it, or change the distance, does that ring move? Does it go away if DOF is disabled? Is it still there with a default CoronaMtl?
Thank you for the reminder, TomG
Glad you got it fixed, and ty for letting us know too! Always handy in case anyone else runs into the same kind of issue, gives a good pointer on where to look for the possible cause