I am so sorry that your Ninebark (Physocarpus) is mildew prone. Sometimes that happens when new plants like the beautiful Coppertina are introduced with more beauty but less disease resistance. Mildew can be controlled by more sun and air circultion but sounds like you have already done that. The copper fungicide is a great solution but since that has not worked I suggest you build up the health of the plant with a compost mulch. A mulch will also help to seal in any mildew spores that have fallen to the ground. A drastic step that could work is to just cut all the branches to the ground now that it is mid spring. (May 5 as I write this) and this gives the plant a fresh start with mildew spores buried under the mulch.  Good luck and let us know how the severe pruning and mulching works out for you. Keep growing, Marianne Binetti