Two things brought this thread to my attention today:
(1) Tomorrow my church begins a series of new electives prior to the service on Sunday; one of them is on FASTING.
(2) An article in the Logos Bible Blog last month was on FASTING:
"How to Fast For God: What Fasting Is, Why it Matters & More" [link]
Reading through the Logos Blog article I noted that the variant texts of Mark 9:29 and Matthew 17:21 that you mentioned are not unique.
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COMPARE Textus Receptus vs. Critical Text
Matthew 17:21 (AV 1873) Howbeit this kind goeth not out but by prayer and fasting. [this verse is excluded from the Critical Text; considered an assimilation of Mark 9:29]
Mark 9:29 (AV 1873) And he said unto them, This kind can come forth by nothing, but by prayer
and fasting.
Acts 10:30 (AV 1873) And Cornelius said, Four days ago I was
fasting until this hour; and at the ninth hour I prayed in my house, and behold, a man stood before me in bright clothing,
1 Corinthians 7:5 (AV 1873) Defraud you not one the other, except it be with consent for a time, that ye may give yourselves to
fasting and prayer, and come together again, that Satan tempt you not for your incontinency.
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Textual scholars conclude that the discrepancies between texts
occurred very early.
All textual scholars agree upon
the numeric predominancy of the readings found in the
textus receptus.
Conclusions differ, many/most current textual critics conclude that these were additions to the text.
Rob