First up, your concept that prophecy has to be understood beforehand is just wrong. Much prophecy is understood only when it comes to pass. For example, the "Abomination that causes desolation" is clearly Antiochus Epiphanese. Who knew that beforehand? The monster was not even born when the prophecy was made.
The confusion about the serpent in the garden is ridiculous. He was not the snake that we know today. Revelation 12:9 and 20:2 refer to the Serpent as the "Serpent of Old, aka the devil and Satan, the deceiver of the whole world. You know that the Serpent is described Genesis 3:1. He was not a snake. His physical form was transformed to what we see today. He still feeds on the "dust of the earth" - that is people created in God's image. That "food" is not physical. Satan needs the cooperation of people to get his will done. He does it by lies and deception and slandering God. Satan is the ultimate user.
I agree that you wont know if a prophecy has been fulfilled until AFTER it has been fulfilled, & you may not know the exact event that the prophecy was referring to until after it has been fulfilled. But at least we know that the prophecy in Daniel WAS a prediction of a future event so it is correct to label it as a prophecy.
It is not at all clear that Genesis 3:15 is predicting anything, other than that snakes & humans will have enmity towards one another, & that was obviously already known before the account was written. So it is not correct to label it as a prophecy.
However if the insistence is that it is a prophecy about Jesus defeating Satan, then the prophecy remains unfulfilled, because Christians claim that Satan is alive & well, & still deceiving everybody. So until it is fulfilled, you can not say it is a prophecy of Jesus defeating Satan, UNTIL AFTER the event occurs, & it might never occur. So the "prophecy" has proven nothing, except that the Messiah has not yet come.
It can also not be proven that Jesus was born of a virgin, & Rabbi's deny that there is any such prophecy of a virgin birth in Isaiah in the 1st place. Prophesying a virgin birth is a useless prophecy, unless it can be proven that the woman who gave birth to the child, was in fact a virgin when she conceived. There is no possible way of verifying this.
The only mentions of a virgin birth are in Mathew & Luke in the New Testament. Paul either apparently did not know of it, or if he did, he didn't think it was important enough to mention to his converts & potential converts. And neither Mathew or Luke can be relied upon on Jesus birth details. Mathew has Jesus being born no later then 4 BCE, & Luke has Jesus being born in 6 CE, a minimum of a 10 year discrepancy. Both authors give totally different genealogies of Jesus, even though they are both genealogies of Josephs line (who apparently wasn't even the father). Mathew has Mary & Joseph fleeing with Jesus to Egypt just after Jesus birth, but Luke has them returning to Jerusalem, & then back to Nazareth after Jesus birth, with no mention of Egypt.
There is also the problem that Christians have created for themselves of explaining WHY Satan was in the garden of Eden in the 1st place. Was God powerless to keep Satan out of the garden (so not all powerful, as Satan is at least equally powerful) OR did God deliberately allow Satan in to the garden to deceive Eve, therefore ensuring that she & Adam would fail?
If Satan was ALREADY in the garden of Eden BEFORE Adam & Eve sinned, then the garden was not perfect or a paradise, & the garden was already full of evil & sin. So it is wrong to blame man for sin. This is a problem for Christians to resolve, not Jews, as Christians are the ones claiming that Satan was already in the garden BEFORE the downfall of man.
