1. Question Restatement:
Mark 7:19 - Does Jesus Really Declare “All Foods Clean?”
2. Answer - Uh .. What in the world?
Regardless of the underlying grammar, or "textual validity" - it is incredibly invalid to make any grammatical inference that is exactly opposite of the argument Jesus said he was making:
NASB, Mark 7:15-16 - There is nothing outside the man which can defile him if it goes into him; but the things which proceed out of the man are what defile the man. 16 [If anyone has ears to hear, let him hear.”]
To Note: The historical issue, here, is the willingness "to hear".
Jesus proved that: if it is true that nothing outside of a person can defile them before God, then it is certainly true that no unclean food will defile them before God, (a categorical syllogism, Butte College).
This alone should sufficiently resolve the question. But, if more is required...
3. The Doctrine of Mercy:
Jesus didn't change anything - he was making the point that it was always the case that man could not be defiled - before God - this way.
It is highly, most, incredibly, a lot, ironic that the form of the answers given here, (and many other places and times throughout history) reflect exactly what Jesus was refuting ...
Paraphrase of Jesus: "You seem to 'think', to rely on reason, to rely on your education, on intellectual argumentation, on traditions, and somehow rationalize some conclusion ... but ... no. Just... No. This is a matter of the heart - and God's love - regardless of how smart and justified you think you are."
In context - Jesus was very explicit, extremely clear, stating that the only thing that can possibly defile a man - before God - is what a man DOES and SAYS.
The intention of both the speaker, (Jesus), and every New Testament writer is is consistent throughout.
NASB, Romans 14:20 - Do not tear down the work of God for the sake of food. All things indeed are clean ...
Even if Jesus did not explicitly state: "I declare all things clean" - it is still the only deductive conclusion possible - based on Jesus' premise that nothing outside of a man that enters him can defile him before God.
Mercy Triumphs over any Judgment under the Law, (James 2:13):
There was a woman ... who reasoned ... that if even dogs merited crumbs from their master's table, and if God's crumbs were awesome, then God would bless her, because she knew that her value before God was at least greater than or equal to: a dog.
And because of her reasoning, and trust in the love of God, Jesus said:
NASB, Matthew 15:28 - Then Jesus said to her, “O woman, your faith is great; it shall be done for you as you wish.” And her daughter was healed at once.
Her faith had absolutely nothing to do with Scripture, but her trust in the love of God - that Jesus was proved and had been showing to everyone.
4. Textual Criticism:
This has zero to do with textual criticism, or Greek analysis. (The issue is "moot" anyway - as there are no authorities to declare the in/validity of the text).
Hermeneutically, regardless of the text, the writer's skill, scribal-accuracy, manuscript questions, etc - "Pragmatics", (Wikipedia link), supersedes any inference made from any grammatical observation, (any grammatical/textual observations must be interpreted in view of a clear and evident intent of the writer).
You could literally take the book of Mark out of the Bible - and still come to this same conclusion everywhere in the New Testament, (even from Hebrew Scripture alone, but that's for another discussion).
This is a simple matter of understanding the heart of everything Jesus taught:
Paraphrase, John 3:16 - God is desperately in love with the World, eternally patient, and infinitely merciful. Trust in his love, that God is 'for you', not 'against you', and prove your hope in this truth by being compassionate and merciful towards each other (because it is the only "just" response); and then, and only then, will you be undefiled/righteous before God. (This is the definition of: The Gospel).
Regardless of any "strange scribal discrepancy" - any argument that anything unclean in this world could defile a person before God is a blatant misrepresentation of New Testament texts, (though James 1:27 does get pretty close).