The origin of the term "fundamental" came about because there were people who simply wanted to identify what was foundational to Christianity. They believed that there were certain fundamentals that were held in common by all who were Christian. However, the key to being a fundamentalist wasn't as just about content; it was also very much about process. It wasn't about being liberal or conservative, though today we might think of some of their fundamentals in this way, they were not playing that game the same way it is played today when they created the term.
Hm, as I understood it the original fundamentalists were very much about not being liberal--as the original group of "fundamentalists" arose in response and reaction to the liberal theology and higher criticism of the late 18th/early 19th century, and while it was certainly an attempt to continue existing evangelical movements and drill into the fundamentals of Christianity, it was also an attempt to reject the use and integration of modern scholarly discoveries and methodologies into theology and Bible study. Therefore the original 5 Fundamentals were no accident:
* The inerrancy of the Bible
* The literal nature of the Biblical accounts, especially regarding Christ's miracles and the Creation account in Genesis
* The Virgin Birth of Christ
* The bodily resurrection and physical return of Christ
* The substitutionary atonement of Christ on the cross.
The original "targets" of the Fundamentalists were the scientific theory of evolution, "nonliteral" interpretations of the Bible (including alternative explanations of Biblical miraculous accounts), and alternate theologies of atonement. Only that last has nothing to do with the direction Christian Liberalism was going at the time (and it's only arguably fundamental, as it excludes vast swathes of Catholic and Orthodox theology--this exclusion was probably not a problem in the minds of those who formulated it, though).
Updated for today, the (Protestant Christian) fundamentalist adds to the above:
* opposition to abortion
* opposition to homosexuality
as markers of orthodoxy. If you don't believe me on this, look at what 2 topics are forbidden to be discussed on most of CF
I can't address the rest of your post because what you and BryanW92 see as progressivism and what I do are probably actually 2 different things (and in fact you may be more familiar with the proper names for movements inside the UMC than I am). I personally identify as liberal but I though progressive was pretty much the same thing. But I am very much not about being doctrinaire. I don't know all the answers enough to force them on everyone else...