You raised a number of important points in your letter and especially the question about the nature of unity. "What", as you say "will oneness look like?" The answer is that there is no real answer. In my opinion the greatest enemy of unity is that old curse, the Law. It rears its ugly head in obvious legalism, but also in more subtle ways. One is through the abuse, or over-use, of leadership. A good parent or leader aims to create freedom and independence. The legalistic leader fails to recognise the God-breathed life in the developing Christian and labours to impose rules, to teach habits and to mould the young believer in the leader's image. That creates groups with distinctive characteristics reflecting their leader's personality.
A similar piece of divisive legalism that is particularly prevalent today is the emphasis on how-to-do-it teachings. In the manner of secular management training, these teachings set out methods and prescribed routines and duties that the faithful are supposed to follow. In most cases the behaviours they recommend are innocent or even admirable, but they are laws. They set on paper the things that God wanted to write on the heart. These joint pressures produce narrow unities that also exclude people. They slice the church into mutually exclusive parties whose apparent unity comes from nominal acceptance of a set of principles or rules set down by the leader or the how-to-do-it course.
The solution to all this is to let go; to believe what God says; to trust that there really is a Holy Spirit indwelling those who have been reborn. In such an atmosphere of faith there is still a place for leadership and teaching, but the driving force is faith, hope and love. Faith that God is alive and true, hope in the sense that we see a future that God is creating, love in the sense that we give ourselves to those who follow us rather than asking them to give themselves to our rules, preconceptions and prejudices.
You rightly point out that history does not record examples of visible unity throughout the church. Such precedents are discouraging, but such testing is designed to produce patience, experience, hope, faith and wisdom (Romans 5:3-5. James 1:3-6). Despite contrary evidence I believe that God can and will bring his people into unity - whatever it looks like!