I think that the one wheel with the screws was a gyroscope of sorts. I would guess that the biggest difficulty is height, and once they acheive that, they can just keep tweaking the setup until they can manage to keep in one place long enough. I'm sure the 4-rotor setup is optimal since both groups used it. However, I still wonder if you couldn't get by with one big rotor and a small counter-rotation rotor. It would have fewer moving parts, and be lighter. You might have to have them working on separate drives though, like feet for the big rotor, and hands for the small one. My read was that they had to stay at 3 meters for 1 minute.Maybe the trick is to hover just off the ground for the majority of the 60 seconds, then drive hard for the height as quick as possible to lessen any chance of drift.
I was thinking that one larger shaft with multiple stacked rotors could be effective also, but it seems like the kind of thing where one team had some success with an idea and everyone else sort of followed suite. Interesting how design often has to become iterative in an inbred sort of way for any real progress to be made. It has to be so edited in order to control it, that there is sometimes no way out of that particular thought stream.I'm sure the 4-rotor setup is optimal since both groups used it.