They do not have the build the wall completely sea to sea. A lot of the land along the Texas border is really difficult and dangerous to travel through. They may only need to build a 1000 miles or so.
As to Indian reservations the tribes if I understand right have their own police
But that's not what Trump has been saying.
Remember folks; that after the Alamo loss, there was an 18 minute battle at San Jacinto.