Yes, I know how a helicopter works. I can, at a push, even describe retreating-blade stall and vortex ring (RAF A2 fixed-wing QFI, so my description might be a bit superficial!).
I believe a quad-rotor can be controlled with the loss of a rotor if each rotor is gimballed, which would allow each rotor to give an element of sideways thrust. The loss of a motor is probably more easily resolved with back-ups. The point is that if it looks economically viable, it will be done. If it looks likely that it won't be cheaper than a single-rotor helo, it won't happen.