OK, by "cannot cooperate", you meant "unable to coordinate communication about their already-shared values" rather than "unable to agree to support each others' unrelated interests". Got it.

I don't see how that's a useful hypothetical scenario. If you have enough secret agents that you can coerce a large percentage of voters to do what you want, what voting system is able to stop you?

"I will help you with your thing if you do the same for me" is the core ethos of non-dictatorial civilization. IMO, QV encouraging cooperation (not "collusion") is a point in its favor, not against.

I don't understand. A 40% group will (and IMO should) beat 3 non-cooperating 20% groups in pretty much any voting system. 

A system that encourages groups to work together for their collective benefit seems like a solution for that situation, not a problem.