I believe Satan possessed or influenced the serpent, but the serpent is not a code word for Satan (with no actual real serpent present).
In fact, the idea that serpent is a code word for Satan (with no real serpent present) does not add up with what we read in the Bible. It would be one thing for Scripture to describe the word serpent as a code word for Satan (with no real serpent present), but it would be another thing altogether to keep up the illusion of this kind of metaphor by God telling us the serpent (Satan) to crawl upon his belly and to eat dust. This is what a snake does today. In addition, Scripture usually has another testimony. If crawling upon his belly and eating dust have some other kinds of spiritual meanings, we should see that in the Bible clearly associated with the devil. But in reality: We learn that a serpent and dust eating is simply referring to a regular animal and not the devil in another place in Scripture.
For Micah 7:17 says,
“They shall lick the dust like a serpent, they shall move out of their holes like worms of the earth: they shall be afraid of the LORD our God, and shall fear because of thee.”
Furthermore, Genesis 3:1 says,
“Now the serpent was more subtil than any beast of the field which the LORD God had made.” (Genesis 3:1).
This means that the serpent here is a created animal here. God says that the serpent was more subtil than any BEAST OF THE FIELD which the Lord God had made. Satan (a fallen angel) is NOT among the beasts of the field which the Lord God had made in the beginning.
Again, Scripture stresses again this is a literal animal in that it places it among the other animals in His creation.
For Genesis 3:14 says,
“And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field;”
For God says that the serpent is cursed above all cattle, and above every beast of the field. So this surely cannot be a code word for Satan (with no real serpent present) because it lists the serpent in being among the cattle and every beast of the field. If we were to go back to Genesis 1, and look up the day God created the beasts of the field, there is no mention of how the angels were created.
Note: Psalms 104 implies that angels were created on Day 2.
So in conclusion:
What happened is that Satan possessed or influenced the serpent (a real animal) to speak (by some unknown means we are not aware of). The serpent was in part responsible for the fall of Adam and Eve because it cooperated with the devil. This is why the serpent (an actual real animal) was punished.