Trump can walk and chew gum at the same time. Keep in mind, all of these mechanics that you’re talking about are only relevant to the extent that Trump has to make good on a threat. He can immediately start building the wall with available funds (it was actually authorized via Congressional statute several years ago), and then really he’d have at a minimum nearly four years (Novemember 2020) to get Mexico to start contributing to it.
Upon entering office in January 2017, he’ll immediately have three million people reporting to him. Via his executive authority, he can divert a few hundred of those to each of two tasks: (1) building the wall; and (2) identifying mechanisms to stop illegal aliens from remitting funds to Mexico. He can also call on his allies in Congress led by Jeff Sessions to start drafting legislation to that effect as well. Then simply place a call down south specifying that they can either agree to pay for the wall or face hundreds of federal employees doing everything they can to stop a flow of billions of dollars in hard cash flowing into their country. I think their choice would be obvious at that point.
So do I. I think they tell him to go pound sand.
There are enormous roadblocks to actually putting an effective stop to those remittances, and even then, you've got to put yourself in the position of a Mexican politician. Knuckling under to Trump's threats would be instant political suicide. There's 0% chance of that happening before the U.S. actually shows that it is truly capable of shutting down illegal remittances, which I don't think they could. The most common workaround would just be using legal immigrants to send the money home.
Look, here's my point. There is a lot of public support for building a wall. It's a good issue, and good policy as far as I'm concerned.
But forcing Mexico to pay for the Wall just sounds like an empty boast to a lot of people, and I think it ignores the political realities in Mexico. National pride simply wouldn't permit them to knuckle under.
There's nothing wrong with making a threat if you're prepared to back it up, but in this case, I think that's where it falls down. The Mexicans will never agree to that (which is why it would have to be a U.S. imposed tax for which Mexican agreement isn't required), and Trump will lose his first showdown with another government. I'm sick of weak Presidents, and I don't want that to happen.