I can see where you are coming from Mitch; I ought to after all this time. The difficulty I have is that although you claim to be defending Protestantism it seems to me to be a re-imagining of Protestantism in a highly individualistic and postmodern fashion. You are seeing a significance in the Reformation which would not have been seen by the Reformers.
Let me have another go at explaining what I think sola scriptura does and does not mean and see how far you can travel with me. It does not mean that that scripture is the only authority in matters concerning Christian belief and therefore it is not the case that each Christian is left alone with the Bible to work out the truth for himself. Rather scripture is the only final authority to which the church can refer when determining what is true in matters concerning Christian belief. Hence when Protestants disagreed they did not agree to differ but tried to find common ground - that is what the Protestant confessions are about.
I also find that like many Protestants I share your dislike of state Churches, ironic in an Anglican living in the UK I know, although her majesty the Queen does become a Presbyterian as soon as she crosses the Scottish border. However I would see the events in the time of Constantine a little differently to you. Constantine wanted Christian unity even if that meant doctrinal compromise - he wanted peace in the state - the Nicene Fathers would not acquiesce any more than the Reformers would go along with humanists like Erasmus who wanted as broad an understanding of Christianity as possible. So it was Constantine who was basically inclusivist and the Fathers who wanted doctrinal precision. The aim of the creeds was not to define Christianity but to define true doctrine. The various heresies which the creeds and definition are designed to exclude are still identifiably Christian heresies. But I agree that it is the consensus of the Church through the ages that gives the creeds authority.