The language of the New Testament is unambiguous evidence for a flat governance structure under Christ as Head.
The five-fold ministers travel, write, teach, equip, frame doctrine, and appeal. They are not operational controllers; they are equippers and persuaders. Their letters carry weight because they teach the truth, not because they hold an office that demands compliance. Where they could command they choose to appeal, and they say so explicitly.
Local elders shepherd the flock that is among them. The congregation submits to them — the ones present, watching over local souls, accountable to Christ for how they keep watch. Practical authority sits where Christ has placed it: in the local body, exercised by local elders who themselves are called to serve and not to domineer.
Congregations are decision-making bodies. They are addressed as adult brothers and sisters in Christ, expected to weigh, discern, and act on what they have received. The Jerusalem Council's "it seemed good to the Holy Spirit and to us" is the characteristic note of New Testament church decision-making — the body acting together under the Spirit's leading.
The one place the structure is not flat is the gospel itself. Doctrine is non-negotiable, and on that point the apostles speak with the authority of Christ — to which they themselves are bound as much as anyone. But everything downstream of doctrine — how a church organises itself, who it appoints, how it runs its gathering, how it spends its money, how it does mission, how it cares for its members — sits inside the local body, helped and equipped by visiting five-fold ministers, never overridden by them.