Re: How many Shuttles could have been kept operative..
It's not just that...
How on earth are you going to patrol the wall? If it's made of steel, a few hours work with an angle grinder is enough to cut through. And I wouldn't put it past some enterprising persons to manage to create a hidden 'door' in the wall that is almost undetectable from the US side*. And unless you're going 'Game of Thrones' high with the wall, it can be scaled.
So wall will need to be patrolled with high frequency. Also, US will be patrolling their side, but can they patrol the Mexican side? (in this respect, makes more sense for the US to build the wall a couple of hundred metres inside the US so they can patrol both sides). But you need to patrol thousands of miles of wall with how many people exactly??
At best the wall will prevent large numbers of people crossing at once, but individuals or small groups could still get across
*Allied POWs in Colditz, Germany's most secure prison camp, managed to build 'doors' in brick walls that were undetectable to fairly close inspection, if the steel wall is in sections, disguising a door would be easy