Would be nice to see a close up picture of how the motherboard is bolted to the heatsink.
We have used AMD, and on those mainboards (don't know if it is the same for Intel) the heatsink is originally attached through a frame surrounding the CPU on the mainboard. This frame is attached to the mainboard thorugh four holes. We just removed that framed and attached the mainboard through these four holes. So, CPU is attached to the mainboard in the usual way, and then the mainboard/CPU is attached through four screws at the aluminum block.
It is possible to fixate the mainboard at some other points, but in our case that was not necessary.
Drawback with this method is that care has to be taken so that there is good connection between CPU and aluminum block (pressure has to be even, and not to high).