I will answer it with my opinion, based on my studies, as to whether Matthew said one thing in 5:32 and another in 19:9. (in a later post I will try to respond to your Church Father quotes)
The two passages both regard the interpretation of the controversial passage Deuteronomy 24:1-4
Well at least that is my opinion. I find it remarkable that the whole sermon on the mount has been deleted from Mark, and that the Matthew 19 conversation has been reworded to show no "exception". But that can easily be understood when one considers that Matthew was written for a hebrew christian audience, and Mark was a redacted version suited for a gentile audience.
(how many Church Fathers do even mention this fact?)
On Matthew 5:31-32
This is found in a part of the sermon on the mount focused on correcting contemporary rabbinic teaching.
In Matthew 5:31 Jesus quotes a saying "whosoever shall put away his wife, let him give her a writing of divorcement".
Jesus corrects this view, but how the correction is made, is probably not at first glance obvious.
"but I say unto you, that whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery, and whosoever shall marry her that is divorced committeth adultery.
First I want to point out, that the word "cause" is the greek word "logon", which we would understand to be relating to some scripture, i.e. Jesus says that except a scripture of fornication applies (see Deuteronomy 22:13-21), the husband will be held responsible ("causes") for any later adultery. The fact that he calls it "adultery" (greek "moicheia"), also denies the validity of the divorce paper, so that is one way in which he rejects the practice of the divorce paper. The original union is still valid - except if following a process related to Deuteronomy 22:13-21.
Now, where does Jesus get this principle of responsibility from? He actually gets it from Deuteronomy 24:4, where the ruling is, that the husband can not take the wife back because he has CAUSED her to defile herself.
OK this is probably not something you will find in your translation, but the point is that the grammatical tense of the verb ("she has been defiled"), the hithpael tense, according to a 16th century scholar Henry Ainsworth (a biography here, with links to his digitalized work, where you can look this up, in his "annotations on the pentateuch"
Henry Ainsworth - Wikipedia)
“she is caused to be defiled”, or “is caused to defile herself”. The gr and chald translate “she is defiled”, but the hebr word Huttamaah is of such composition as implieth both the defiling of herself and the cause thereof by her husband who put away first.
So my final conclusion: the "except" in Matthew 5:32 IS an "except" ("parektos"), and it regards the law of sex before marriage and the obligation to kill the woman if she is not a virgin - see Deuteronomy 22:13-21.
On Matthew 19:7-9
First I should explain, that I read Matthew 19:3-12 as a structure of 3 question-and-answer substructures.
The question being raised in Matthew 19:7 "why did Moses command to give a writing of divorcement and to put her away".
I will have to say, that this question shows a severely distorted reading of the Deuteronomy 24:1-4 passage. Moses certainly did not command neither. The structure if Deuteronomy is an "if and if and if and if and if then ...".
But anyway, Jesus brings in another "I say unto you". This is where we will expect to get the correct interpretation of the questioned passage,
"and I say unto you, Whosoever shall put away his wife not over fornication, and shall marry another, committeth adultery, and whosoever marrieth her which is put away doth commit adultery"
Agian, for me it is simple to understand, that "not over fornication" refers to a divorce according to the rabbinic practice derived from their reading of Deuteronomy 24:1-4. That is, any practice of ending a marriage or betrothal not using Deuteronomy 22:13-21.
One more detail, while we are at it. In Matthew 19:10 the disciples ask Jesus "if the case of a man be so with his wife it is not good to marry". Actually the word for "case" is the greek "aitia" (Strongs 156), which means "cause" or even by implication "accusation". Can you answer me, why do the disciples suddenly begin talking about a "cause" / "aitia" ? I think this has reference to what I mentioned with respect to the Matthew 5:31-32 passage, namely that the man "causes" or "will be accused of" adultery.
So in *conclusion*, after all these considerations, do I consider the "except" in Matthew 5:32 and the "except" in Matthew 19:9 to be the same? No, in my interpretation they work not exactly in the same way.
And I have already posted earlier in this thread a link to some research into the difference of "not over" from "if not over". And I will repeat it here for the interest of clarity
Reasons Mὴ Eπὶ (Mh Epi or Mē Epi) Should Not Be Translated “Except For” (Mt. 19:9)
Timothy Sparks has more relevant stuff on his page, but ok I will spare you for now - if you are still alive after a 2-page post (!)
(I will try to make one more post covering your quotes of Church Fathers)
regards. Peter