I measured 4.17mm clearance from the mainboard base to the top of the cooler, which is the tallest component on the mainboard which must clear the underside of the top cover.
Must have M key, but can accommodate B+M.
That’s an M key, which makes sense (PCIe x4): M.2 Interface, Key and Socket explained
The signalling is PCIe, not SATA. So no B keyed devices unless they also have the M key.