If you're talking about the DACA people, there are only about 800,000 and since they can't reasonably be held responsible for what their parents did, they are a special case and special arrangements should be made for them if this does not appear as an invitation to future illegals. Trump wants to balance the damage allowing them to stay will do with the border wall to protect US immigration laws. So far he had done a spectacular job in reducing illegal immigration, far better than anyone would have thought possible, so a reasonable person would give him the benefit of the doubt on this issue.
Not for 25 billion dollars. Half the illegal immigrants in the country came by motor vehicles, plane, and boat. Haven't seen a wall yet that would stop, a plane, boat, or vehicle at a legal boarder crossing. Once illegal immigration adapts as it always has, the wall, will be a 25 billion dollar environmental and economic monument to Trump's stupidity. There are several places near major cities and towns where a wall would be effective. However, in most places a combination of 21 foot fencing or electronic fencing, and increased patrols would be far more effective.
Even without the wall, Trump's tough talk and tough actions have reduced illegal immigration from Mexico by about 2/3 over Obama's last year in office. The wall will reduce it even further, and then there will be a push to get every business to use E-verify for each hire, which will lower it even further. The key to this strategy, remarkably successful so far, is to make the US inhospitable to illegals and so difficult to get into that few will try.
The number of illegal immigrants in the country increased by 80% between 2000 and 2007 where it peaked at about 12 million. Since then it has been falling rather steadily. The source of illegal immigrants has been changing also. The number of illegal Mexican immigrants have fallen sharply and now are no longer the majority. The biggest increase in illegal immigrants since 2005 have come from Asia, Central and South America and these immigrants will not be stopped by a wall.
Back in 2005, it appeared that a wall would be a real solution to stop the hoard of illegals pouring across our southern boarder but not today because most illegal immigrants don't sneak into the US across our southern boarder. They enter through a legal boarder crossing or enter illegally by boat, planes, or across our northern boarder.
Yet the fact remains that during Trump's first year in office illegal immigration across the Mexican border declined by 2/3 over Obama's last year in office and the total was the lowest since 2000. This startling decrease over Obama's efforts was due entirely to the tough talk and tough actions by Trump that persuaded potential illegals that the US would be an inhospitable place for them.
Obama, with the full support of his Party, sent a consistent message to illegals that if they could manage to get across the border and away from it, he would do his best to prevent them from ever being deported unless they committed a serious crime, so no matter how many time they were caught near the border an deported, they had his word that if they made it a little further from the border the next time, he would protect them, in fact, he would encourage states to make them de facto citizens. President Trump, on the other hand sent the message that if they entered the US illegally or entered legally and remained illegally he would hunt them down and deport them so they would never be safe. That's why 2/3 fewer illegals attempted to cross the Mexican border during Trump's first year in office than in Obama's last year in office.
As you point out, not all illegals come across the Mexican border, but in 2016, an estimated 400,000 did successfully cross the border and evade detaction (a slightly larger number were stopped or caught and deported), and this was a 23% increase over 2015, so illegal immigration from Mexico is not an insignificant problem. A smart barrier, such as Israel built on its border with Sinai, will reduce the number of illegals from Mexico to near zero. Before Israel built that barrier, tens of thousands of illegal immigrants from east Africa were flooding into Israel. Since the barrier went up, none have successfully crossed the border.
If you make America hospitable for illegals who are already here and you don't spend the money to prevent more from coming, as the Democrats propose, the numbers who come here illegally will increase, but if you make America inhospitable to illegals who are already here, the numbers who try to come illegally will decrease as President Trump has demonstrated. If addition you also build a barrier as effective as the one Israel built the number of illegals who successfully cross the border will be reduced to nearly zero.