I uploaded the sch for the mini backplane.

Not a lot to see here.

The difficult part was to figure out the signal assignment through the MiniSAS link.

MiniSAS HD is an SNIA spec, namely SFF-8643. Standard documents have tables detailing how PCIe can be routed over the cables and connectors of the interface. I bought a PCIe-MiniSAS HD adapter board on Amazon but that didn't follow the pinout in the SINA documents. These kind of adapter boards are used for storage devices, so I'm sure there's a reason why their signals assignment is that way, but I couldn't make sense of it so I had to reverse engineer based on the hardware.

Yet another difficulty was that on the adapter board the PCIe Reset signal was seemingly not connected. Maybe it's not needed for the storage devices or I just got two faulty units? I ended up populating one of the empty resistors with 470R.

I overlapped photos from both sides of the card to trace back which PCIe line goes to which MiniSAS pin.