Here is another argument someone might use to escape these facts:
I entirely agree that the church should be one in formal unity throughout the world. As a matter of fact, it is. All the true churches in the world are indeed in full communion with each other under mutually-binding councils. The OPC, the PCA, the RPCNA, the FCC, the FCS, etc., are all united together under the possibility of a unifying worldwide ecumenical council. So there really is no denominational separation, and therefore the denominations do not reject each others' de jure legitimacy and authority, nor do they accuse each other of schism.
OK. So if all the denominations of true churches on the earth are indeed united together under mutually-binding councils, as presbyterianism requires, then why is there so much division among the churches? Why so many differences in faith and practice? Why hasn't a mutually-binding council been called to clean this mess up?
Well, although we are under common councils, yet those councils haven't met for a long time. It's been ages since there has been a full ecumenical council, for example. But the possibility is still there, so that's OK!
If this is truly the case, then I must say that all these denominations have been severely neglectful of their duty. It is the duty of higher assemblies to help discipline and correct the lower assemblies. If a session goes wrong in doctrine or practice, the presbytery is to step in and bring correction and discipline to fix the problem, and so on. So if all these denominations are really united together, then they have shown quite clearly by their actions that they don't really care at all about the purity of the catholic church. They care about purity within their own little groups, but they don't care enough about the rest of the Body of Christ to do what needs to be done on a broader scale. When a liberal denomination professes to believe a certain confession of faith, but in practice completely ignores it and has no discipline whatsoever based on it, we rightly say that their profession to hold to that confession is a mere pretense. A church with no discipline at all is a church that is not truly bound to what they say they hold to. Well, I would say that the same thing would apply here. A church which professes to be concerned for the unity and purity of the whole church and that it is quite possible to call a council of the whole church to deal with problems in these areas, but which never actually calls any councils to deal with these issues, shows that their profession is mere pretense. They are not really unified; they merely profess to be unified while living dis-unified in practice. So I would say that their profession makes no real difference and does not describe what they really are or what they really believe.
But, in fact, the various Reformed denominations don't claim to be part of one unified church under mutually-binding councils, so the whole argument is moot anyway. The OPC (Orthodox Presbyterian Church), for example, acknowledges that the Reformed denominations exist in separation from each other and not in full communion (not even such a lazy full communion as described above):
E. The present division into separate denominations is because of unfaithfulness to God as expressed in beliefs, teaching, and living, on the part of both individuals in the church and the churches that are contrary to the Word of God.
F. We find ourselves in this sinful situation as we undertake to pursue the mandate to unity. There exists between us and all other churches a sinful disunity that demands reconciliation in a biblical way. This sin must be faced and removed so that true and full unity and fellowship of the church may be reached. (Biblical Principles of the Unity of the Church)
The visible unity of the Body of Christ, though not altogether destroyed, is greatly obscured by the division of the Christian church into different groups or denominations. In such denominations Christians exercise a fellowship toward each other in doctrine, worship, and order that they do not exercise toward other Christians. (OPC Form of Government, Chapter IV)
The FPCS (Free Presbyterian Church of Scotland) points out the same thing. In answer to the question, "Why are you a separate church?" the FPCS responds:
We take the Bible alone as our guide to Doctrine, Worship and Practice. We preach the Gospel of God's sovereign and free grace. . . . In order to safeguard these precious truths, we are separate from other Presbyterian churches which have compromised on these matters. (Frequently Asked Questions)
Later in the same document, it adds:
Being a Presbyterian church we believe in the unity of congregations in a Presbyterian structure. We do not believe in the spurious unity of the modern ecumenical movement which minimises doctrinal difference between the Protestant churches and which is leading towards re-union with Roman Catholicism under the pope of Rome. We believe in the unity of all Spirit-taught, born-again, believers in Christ throughout the world, and that they ought to be united in one Presbyterian Church.
Therefore, the inconvenient fact remains: Divided Presbyterian denominations, if they are truly presbyterian, are rejecting each others' de jure legitimacy and authority and accusing each other of being schismatic. There is no other way to read the situation without abandoning presbyterian church government.