Brief status update:
After the advise to keep going with copper I've gradually raised the level further from 0.6 to 0.72 and kept it at that level. In addition I've been keeping a close eye on the hippo tang since she is showing most signs.
On 2nd of september not much has changed. I took a swap from the glass wall of the QT and found it was infested with ich:
After that I've taken the fish into 3 separate buckets, to scrub down the complete QT including equipment with Vinegar and bring the copper level back up to the 0.72 range before placing the fish back. In addition I added a dose of 2 ML of Formalin 37% After that I started to see a decline in numbers.
Today, even though there are still clear spots on the Hippo, the amount can now be counted. In addition I've done a new skin scrape today as well. The main difference I noticed with this scrape is that I had to look around a bit, whereas in the previous scrape they were all over the place. As the copper treatment was there for a while now, and the raise didn't seem to have impact I suspect the real reason for improvement to be the formalin dosing and scrub of the tank.
26 Aug skin scrape
08 Sept skin scrape (zoomed out 1.5x in comparison to above)
I think the prolonged duration and heavier medication has gotten the best of them. 
