zeeoo,
I think the airfoil effects could help, but if you designed for auto, would that rob powered flight payload? If so, the extra few percent rate of descent might not be worth the loss of efficiency.
The number of blades is not itself an issue, the solidity would be, I think. It gets harder to make a 2 bladed system as weight-efficient as a 3 or 4 bladed system, once the needed aerodynamic blade loading is achieved. Large 2 bladed helos are a nightmare for weight .
Also, vibration in forward flight favors 3 or 4 blades, since the "root shears" are very much driven by the number of blades. The texts have good discussions of this.