Could you post a screenshot of that fringe problem, so i can better understand? In A7, theres an energy conservation mode, that actually clamps result of reflection + diffuse when it exceeds 1, while other renderers balance the reflection vs diffuse scale uniformly by scaling down diffuse as soon as reflection starts to appear and vice versa. New energy conservation is available in daily builds, while Alpha V7 still uses the old one. My point is that old energy conservation produces unnaturally strong reflections at grazing angles, so that could be at least a part of your problem.
Other part of the problem could be tone mapping. I found that most of the backplates are at least slightly tone mapped by the camera, while HDRI maps are not (they must not be to work correctly. And when i was doing my Mercedes Benz renders, i found out that one of the biggest mistakes i did was i kept my car linear, while backplate had some slight tone mapping, so i had strong reflections around the edges of my cars and it did not fit the backplate. So it may be a good idea to set highlight compression to at least 2. And of course, backplate plugged in the shadow catcher and direct visibility environment override should be plugged in through the CoronaMapOutput node, so that it is not affected by color mapping, including highlight compression :)
As for the other note about motion blur:
1, set ShadowCatcher projection mode to Do Not Alter, this will make SC use input UV coordinates.
2, plug your backplate through Camera Map Per Pixel node into backplate slot
3, chose static camera as Camera Map Per Pixel projection camera
4, use different camera as the moving one
The thing with Corona is that fakes are often penalized by performance drops.