Interesting - if you enable the Development/Experimental stuff and change the Solver in the Light category to anything other than Auto (except Photon, which crashes to desktop), then the artifacts go away. GroupNew or Scalable would be possible choices for instance.
We'll take a look into what is happening (internal, 310521345), meantime you might try changing the Solver to see if it lets you move forward with the project.
EDIT - PS, you definitely don't want to have the HDRI in the Diffuse of a Corona Material. You must use a CoronaLight material, and drop the HDRI into that. Doing that doesn't resolve this issue, but it is the right way to do it :)