1. We can build it in Mexico. They are militarily weak, so if we need to "take" a piece of their land on the other side to build the fence we can do it. It's to stop an invasion ~ and is morally justified.
2. We can build a fence on OUR SIDE that stops mass movements and at the same time patrol between the river and our fence to take care of saboteurs and problem people. Just put formal entry/exit points along the fence and you can keep using it ~ and if some idiot decides he has to take his cows to the river to water them, we'll be waiting there for him ready to have a barbeque! Those cows aren't going to run far with that fence in the way.
To your first point, you can buy the land, so Mexico is compensated.
However, the problem with trying to force people back is, what are you forcing them back to?
If I were an illegal alien, and my choice was a very difficult life or to jump off a cliff to certain death, do you really think that by making my life difficult or impossible that I’m going to jump off that cliff?
To say that we “cannot” would not be techincally correct if we consider that we have people living on the Antarctic and we put people on the moon. I understand that.
I hadn’t seriously considered violating the border but that would solve the other physical problems of a fence. I don’t think you could get that over the political hurdles to make it happen.
To me it seems that far too many people rely exclusively on a fence without giving any thought to the other problems allowed and created by the lack of federal interest.
IMO, a fence as you describe should be an absolute last resort with all else has failed, not the first option.