You choose your quorum trading off cost/complexity vs risk-tolerance. You ensure that not forming a quorum is impossible in scenarios that you care about. e.g. You may decide it's OK not to form a quorum if the entire USA power grid goes offline.

The broad problem is that you're trying to apply the mathematical proof of the CAP theorem to the real world. For example, the proof of the CAP theorem treats single-node failures as a case of network partitioning, which is logically elegant. But in the real world, it's just not realistic to consider a dropped TCP connection as equivalent to the failure of a datacenter, as you seem to be doing.