[yock]

Regarding a, how do you figure stall speed is higher? Stall-spin accidents occur precisely because aircraft are in a bank near the ground. They're more common on landing than takeoff because procedure often dictates flying the runway heading until having sufficient speed and altitude to recover.

Regarding b, wind direction is still important. Your airspeed would be constantly in flux when negotiating a landing at a bank since your angle to the wind is constantly changing. This is especially perilous at low speed near the ground due to the risk of spinning.

There may absolutely be benefits to a circular runway, but I see a lot of increased risk around inducing low altitude spins.