The problem Mulvaney alluded to is the Law invites trouble.
His example was apt that the Wall is subject to the river bed. In some places the natural flood zones cause any barrier to be placed deeper into the US, even as far as one mile back from the river.
The Law still says that You’re in the USA when “one foot is planted on American soil”, thereby the illegal is looking for Border Patrol to catch, so then the existing immigration leniency laws are in effect.
The Ports of Entry are evidently are now authorized to return the invaders back, to wait asylum in Mexico. The wetbacks who cross the River are treated differently.
I believe I read that Trump had decreed that if a person enters illegally, their asylum claim can’t be processed. They have to go back and enter through a legal port of entry. That may be held up by courts, but he would eventually win on that.