With an early start from Huehuetenango you should have no trouble getting through this border point and onward to San Cristóbal de las Casas, Mexico. From the bus terminal, it's a hike of about 1km to the Guatemalan immigration post. Mexican immigration is at Ciudad Cuauhtémoc, 4km west of the border; taxis charge 10 Mexican pesos for the ride. About 30m past Mexican immigration, you'll find TAOSA buses to Comitán (50 pesos, every half hour), from which there are onward connections to San Cristóbal (55 pesos, 2¾ hours) over a road that is currently under repair.
The strip in La Mesilla leading to the border post has a variety of services, including a police station, a post office and a bank. There are also money changers who will do the deal – at a mediocre rate, but as no banks on either side of the border will perform this exchange it's your only option. For changing dollars, you're better off going to a bank in Huehue or Xela.