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Okay, I did my survey. Before I give my results, I want to explain my methodology.
The famous 37-volume set of the Church Fathers was translated in the 19th century. I have not done a complete study of the men who did the translations, but I believe that for the most part they were Reformed or Presbyterian theologians and scholars. When translating the Fathers’ citations from Sacred Scripture, they tended to use the wording from the KJV. Therefore, I picked two text strings that are unique to each of the passages in question: “in the very act” and “and sin no more” from the John 8, and “upbraided them” and “take up serpents” from the supposed extended ending of Mark. I also expanded my search to all 9 volumes of the Ante-Nicene Fathers.
Here are my results, which are pretty meager.
I found only one citation of any kind from John 8:1-11, and that was only the phrase “go and sin no more,” quoted completely out of context in Section 6 of Pope Callistus’ 2nd Epistle, which was written to the bishops of Gaul.
Of the “longer ending” of the Gospel of Mark, I found three citations (only three citations in over 9,000 pages of material!).
Mark 16:17-18 was quoted in Book 8, Section 1, Paragraph 1 of the Constitutions of the Holy Apostles, which is a compilation of teachings from various sources.
Verses 15-18 were paraphrased in Chapter 14 of the Gospel of Nicodemus, from the Apocrypha of the New Testament. Not exactly a reliable source.
And last but not least, the whole disputed section of Mark 16 was quoted in Section 55 of the Diatessarion of Tatian, a harmony of the Gospels.
Back to the OP’s original question – if we were trying to reconstruct the entire New Testament from the known writings of the Ante-Nicene Fathers, the pericope about the woman taken in adultery would be completely missing, while the “longer ending” of Mark would still be questionable.
This has been fun. We’ll have to do it again some time.