Hummm....well, I think you have a valid point regarding an implied exemption in the First amendment, although clearly not everyone will agree. As far as what is historical dating back to Rome, if we're to be consistent with complaints regarding the Supreme Court looking to Europe for precedents, it would be equally wrong to look to Rome or any subsequent events external to the US for reason to apply a tax exempt status to Churches. I believe you are correct regarding LBJ being the initial culprit in setting the groundwork to undermine the current status, however I disagree that the churches will be under the control of the government were they to lose their tax exempt status. It seems to me that the most commonly used power to control the church has been the threat of losing their tax exempt status...if they renounce or lose it then that control is no longer in place and they would be free to speak truth to the nation. Of course the ACLU would seek other avenues to silence the Church, but they would have lost their current weapon.
Just look at the US then. Look at all 50 states and see that all of them exempt churches from taxes including property tax. The US federal government has never taxed churches, but they are now using this registration issue to silence churches.