> Luckily we already know that we can reconfigure
> for [...] three X4 PCIe configurations.

I just discovered that the only configuration providing x4 x4 x4 ('0x14') doesn't provide any lane for SATA, so that would leave us with either x8 ('0x10') or x4 x4 x1 x1 ('0x2'), which both have all two SATA controllers connected (and use only 16 lanes overall out of 18 lanes).