Yep the 1x riser into the 16x slot.
You can disable onboard graphics and plug monitor into the graphics card connected to the 16x slot.
you can mine with however many PCIe slots you have 16x and 1x.
Just monitor power draw on the PSU!
Looked at the pic again, I only see 1 x 16 and 5 x 1 PCI Slots?
Yeah, you are right, there are "only" 6 PCIe slots on the board.
So one question is still unanswered if I'm correct: do I have to plug into the MB the two 12V PCIe connectors or is the power enough what the riser gets from the PSU? (and the 8PIN EVGA additionally into the GPU)
Why is monitoring the power draw necessary? Can I do something about it to reduce?
A riser should be connected like this. It needs power through a 4 PIN MOLEX Sata cable from the motherboard to the riser plus you need to also connect the additional 8 PIN PCIE cable from your EVGA. Put this riser in the X16 slot like it's already been told and you should boot with the screen showing everything.