Good points. I was looking at a topographic map again earlier today and the contour levels are marked in meters, so the height didn't quite click in my foot/yards American mind right away. 2500 meters is definitely high ground. I had also read one of the histories where they made the point of how two companies of US Cavalry got separated in mid-march, because of the blizzard white-out. Blown snow, mixed with sand.... That was only a couple of weeks prior to these events and 50 miles to the west.So, in case that would be necessary they could have gotten water from melted snow?
Also, would be possible that the Villistas even if they were forced to withdraw that they might have let behind some men hidden to watch to the American position...
You're absolutely correct about getting water from the snow. The water treatment for creating potable water from other surface sources was just in transition. In camp, they'd be continually setting aside water in large kettles to settle out sediment, then it might be boiled, or run through a Lyster bag with calcium hypochlorite - both were very slow processes. Chlorine or Iodine was also used in the field.
The likelihood of an ambushing rear guard on the other side of the pass should be expected. Both sides used that tactic historically. The major bug-bear for Villa's soldiers was they were chronically short of ammunition, as they were cut off from Mexican government sources. Their raids on Constitutionalist bases were sometimes a calculated expectation of reaping more food and ammunition than they expended getting it. Prior to the Columbus raid, they could also tap into ammunition and gun purchases from US sources. After Columbus, that was more difficult, but was done (money talks...) Basically, they couldn't risk many drawn-out battles.
Many of the Villistas were combat veterans by this point of the Revolution, but with a fair amount of backfilled replacements (of varying levels of commitment). The heavier number of replacements needed after the disasters of 1915. Some of Villa's officers were professional soldiers like Col. Felipe Angeles - French-trained artillery expert. Others had less formal military training, but long service in combat leadership, like Cervantes and many others. Arguably the best tactical General on the continent at the time was Alvaro Obregon - Villa's one-time ally, but now his nemesis. Obregon was another with no formal military training, but one of those natural soldiers who picked up modern tactics on the fly and he understood how to make them work in most situations.
*edit* Sorry for pontificating there....