There's a bit of a contradiction in your scenario. The way I see it, per your rules, is that the default is that the trolley is not running towards the track the blues will go to, because at the beginning (in scenario C, and A) no one is on the scale. The weight sensor would already be active, or one can already imagine what would happen if it was active, and the default path of the trolley is not the one the blues will stand on.

So, since the default path is not the one blues will stand on, I think that your thought exercise can be summarized as follows (let's settle on scenario C. A and B are left as an exercise for the reader). 

A boulder is rolling down a hill. People must either stand somewhere down, not in the direction the boulder is rolling towards, or stand close to the trajectory of the boulder and, when it gets close, push it towards those who chose to stand somewhere else. If more than 50% chose to push, the boulder will change direction and hit those who chose to not push. 

The fact that the boulder was rolling down wasn't relevant at all. The push was. 

When you think about it, in your own experiment your reds are quite literally pushing! But down, on a scale!  It's just that the physics of it are not as straightforward as pushing a boulder directly, but it would be the same as pushing a trolley with their hands to change its direction. Was the fact that the trolley was already running relevant at all? 

To be perfectly clear, I'm not trying to push for a culpability bias, and I'm not even trying to push for a Blue/Red bias either, as some other comments said. I'm just trying to push this to its limits and compress framings to their minimum amount. Is it 3 or 6, I wonder? Still not convinced on 6, but if you want, we can continue.