Very nicely laid-out.
But the issue here is that the wall is not the wall. The wall is a symbol for white people who are losing both numbers and opportunities, and they are turning to a centuries-old American practice of blaming darker-skinned people for their troubles. The wall has nothing to do with actual immigration policy — it has to do with racial scapegoating: all those “Mexican rapists and drug dealers.” The boogeyman.
Funding for the wall isn’t about building it. It’s about “winning.”
For the people who support Trump, it’s about validation: validation of their lives, of their prejudices, of their power in numbers, their right to continued systemic privilege over darker-skinned people.
For Trump, and the Congressional lint that clings to him, it’s about salvaging face after falling face-first in the mud: the mid-term election, the continual unravelling of the top levels of the executive branch, and the Mueller indictments. Trump needs to win in a show of strength.
As you point out, five billion isn’t close to enough to build a wall. But it is enough to count as an outrageous request, and to signal dominance. Trump is telling Congress that he is going to make them submit. His base wants to see that happen.
The fact that the wall is actually a stupid idea actually helps with that.