Here is a quote from William Safire. I have found that he is almost never right about politics and almost always right about the English language. Note that he shares your view about "almost unique."
Most people take the noun perfect to mean "flawless, faultless, impeccable," as in the King James translation of God's description of Job as "a perfect and an upright man." In that absolute sense, the Founders would seem to have been mistaken to modify the noun perfect with more; either something is perfect or it is not; you cannot say "more perfect" any more than "more unique."

The primary meaning of the noun, pronounced PERfect, is "complete, whole, finished," and the verb taking that action, pronounced perFECT, means "to complete, make whole, finish." That is why "to form a more perfect Union" is correct; the Founders (formerly Founding Fathers, but that leaves out Abigail) were working toward the completion of the Union and not claiming to be able to finish, to perFECT, the work.
My Random House College dictionary has cwm, crwth, fjord but not strewth. My impression is that the w in twice is not a vowel, because you have to use your lips to form it, but I defer to anyone who knows.