Ashes,
I have also heard the betrothal explaination and have a hard time accepting it. Daniel Kaufman writes of it in Bible doctrines which as you probably know is held in high esteem by most conservative Mennonites. I am not sure exactly where I fall on the Matt 19:9 passage because the writings of the early church opposed remarriage. Again, this is not to critisize anyones choices in this matter. There is just enough doubt in my mind that I feel lead to remain unmarried.
I wouldn't dream of trying to sway anyone one way or another on the subject. Through my studies, I discovered that early editions of Kaufman's teachings don't contain the supposed belief that divorce was only for the betrothed not the married in cases of adultery. It is only later editions that have that.
Also, as is typical for the writers of the very restrictive anabaptist doctrines, he was a convert later in life, coming out of a liberal tradition. Generally, these men created doctrine that borders on legalism, such as Ron Border's writings of today. Ron, as you know, converted to Amish in his 30's, coming out of a godless background of all sorts of immorality, and joined the Amish because he needed the structure and rules that they live by. Kaufman's story is very similar.
Before Kaufman's book in 1898, traditional anabaptist doctrine allowed for divorce in cases of repeated and unrepented adultery (remember, there were no treatments for STDs back then, if a man whored around, his wife was going to get the disease, too, which would harm the family, so divorce and the threat of divorce was a practical method of controlling immorality) and in cases of divorce before coming to faith in Christ.
So, in Anabaptist history and doctrine, the idea that divorce was forbidden under every circumstance is a johnny-come-lately doctrine.