After the Fall, was the protoevangelium transmitted to all men so much so that all men have knowledge of the protoevangelium even if that knowledge was distorted?
Every mythology has something analogous to the protoevangelium. Also, everyone descends from Adam and Eve, so Adam would have told his offspring about the protoevangelium.
Your question can be interpreted 2 ways due to your not being clear what you mean by "protoevangelium":
In Christian context, "protoevangelium" = Gen 3:15, encoded in the story as God's promise to Adam that someone from his offspring would defeat the serpent, that later Christians would interpret that someone as pointing to Jesus, God's incarnation.
The second meaning (mythological context) implied by your saying "every mythology has something analogous to the protoevangelium" is not common, as I don't think in studies of mythology they use that term. At any rate I try to understand what you mean as: "implicit knowledge" in humanity as seen in several mythologies and religions that feature gods taking human form to do something good for humanity.
It's very important to distinguish the role of "knowledge" in these 2 meanings of "protoevangelium", and not to conflate the two roles. Christianity takes Genesis to be the inspired word of God, so has a very high level of certainty in Christian theology. But "implicit knowledge" in mythologies featuring incarnation are speculative, even the gods taking human form are not necessarily nice. So this "knowledge" is by no means clear, and it's not as clear as performing the kind of salvation that Jesus did for humanity. This kind of knowledge serves more like instinct rather than prophecy.
In the first, it is God's revelation to the writer/redactor of Genesis, which has a limited audience (the Israelites), making the answer to your question needing to be framed in terms of history of the interpretation of Gen 3:15.
In the second, yes, one line of apologetics that C.S. Lewis used was based on a common mythological theme, suggesting that there's an "implicit knowledge" preparing the people post-Jesus to respond rationally to the REAL and TRUE story of how Jesus Christ is God's Incarnate, thereby making Christianity not an otherwise outlandish claim, even when this Incarnation suffered for humankind (because this is another common mythological theme). See Lewis's essay Myth Became Fact published in a collection of essays God in the Dock: Essays on Theology and Ethics (archive.org scan here).
The rest of this answer takes Meaning #1.
I doubt that if we study the history of interpretation of that verse up to when Jesus was born, they linked that verse to one of the roles of the Messiah. If not even the Jewish scribes and theologians had that idea, how could the rest of pre-Christian mankind (who barely knew Genesis) consider that verse a protoevangelium for the whole humankind? Even the extension of Judaism to the Gentiles was controversial shortly after Jesus ascended to heaven.
Rather, the operative protoevangelium for Gentiles at around the time Jesus was born was that the Noahic covenant is God's provision for the Gentiles, and Noahide laws was God's light to the Gentiles for them to know right and wrong so they could also walk on the right path.
But you speculate:
Everyone descends from Adam and Eve, so Adam would have told his offspring about the protoevangelium.
you're assuming that Adam himself understood Gen 3:15 as Christians understand it today. If so, yes, there was distortion. But Christians read Genesis from the final redactor's point of view (the final text IS what's inspired), so if the final text was written around David's time, according to a scholar's estimate, Gary Rendsburg, who defended his theory here, then we need to find out the final redactor's understanding from that era (around 1,000 BC).
Also remember that Jesus himself is the one most likely to supply the hermeneutical key to interpret many passages from the OT, cf Luke 24:27 (NIV):
And beginning with Moses and all the Prophets, he explained to them what was said in all the Scriptures concerning himself.
and that teaching was then transmitted to us via the Apostles and the authors of the New Testament.
"Was the protoevangelium transmitted to all men?" Answer: No.
"Was the knowledge of protoevangelium distorted after the Fall?" Answer: Probably no, because the final redactor of Genesis wouldn't have the understanding of Gen 3:15 as protoevangelium.
Ascribing Gen 3:15 as protoevangelium of the full gospel of Jesus Christ should be seen as later interpretation by Christians after God has taken on flesh in Jesus, who also descended from Adam in the flesh.
