The land drains into the river. If you build a wall, it blocks the water draining to the river, backs up and floods the land.
Yes, the drug cartels crossing the river would be targets.
Yes, I blame them. We don't need Border Patrol sitting in their AC offices and vehicles, they need to be doing patrols. That's their job.
No, it's not against the law for our troops to guard our borders. That is actually why we have troops to begin with.