The techniques the liberals used are outlined in the books Crossed Fingers by Gary North, who is a better historian than he is a futurist. The liberals first aligned with conservatives in the Presbyterian Church who opposed strict subscription to the Westminster Standards. (Remember that Billy Sunday and William Jennings Bryan, who were essentially revivalists and "free will" salvation advocates, were Presbyterians.) In the early 1900s, there were two strains of conservatism in the Northern Presbyterian Church: traditional five point covenental theology Calvinists and what could be called proto-dispensationalists. (Many of the founders of Dallas Theological Seminary and Philadelphia College of the Bible, the first dispensational seminaries, were Presbyterians.) The Northern Presbyterian Church merged with the less Calvinistic Cumberland Presbyterian Church and other small groups, further diluting the power of the strict subscriptionists.
By dividing the conservative forces into two camps, the liberals slowly gained ascendancy. Church discipline in matters of heresy had become a dead letter by the 1920s, as over a thousand Presbyterian ministers signed the apostate Auburn Affirmation evidenced. The final assault on conservatism, either strict subscriptionist or proto-dispensationalist, occurred in the 1930s, when liberals took over Princeton Theological Seminary, which had been the center of conservative Reformed theology for a century, and the General Assembly defrocked J. Gresham Machen and his supporters, the remaining stalwarts of conservative theology.
After the liberal coup, which was completed in 1936 after over a half century of subversion, the remaining conservatives either left the denomination or (in most cases) kept their mouths shut so they could get their pensions in due course. As for the Southern Presbyterian Church, the liberal subversion took longer and was less complete when that denomination merged with the Northern church in 1982. However, many Southern Presbyterians left well before the 1982 merger to form the Presbyterian Church in America (PCA), the second largest denomination named Presbyterian in the United States.
Humanly speaking, the mainstream Presbyterian church is a lost cause and has been so for nearly a century. Granted that today's left wing in the PCUSA is far more culturally radical than their predecessors, they nonetheless remain the heirs of 19th Century theological liberalism which has had the seminaries and most of the presbyteries in its death grip for longer than most of us have been alive. "Mother, child, and womb" or whatever stupid formula is used for the Trinity has its roots in the skepticism and anti-Biblicism of German academia of 150 years ago.
Frankly, I am more worried about preserving the Reformed and Calvinistic witness of the PCA and the conservative Reformed mini-denominations from heresies like those promulgated by Norman Shepherd and Steve Wilkins, who advocate doctrines like baptismal regeneration and works related salvation. I am also worried about evangelicalism in general from the threats of "seeker sensitive" theology that de-emphasizes the evils of sin and the need for repentance and anti-intellectualism in general, where feelings become more important than facts. We have enough to do to fight the subversion of the evangelical church before worrying about long lost causes such as the mainline denominations. We are fighting in Fallujah today, not in Flanders or the Shenandoah Valley.
Thanks for your informed opinion. I wish you well.