I am assuming that after seeing some of the habitat photos, that a "cut" is where the road cuts through the earth, forming walls along the side of the road? I can see how attractive it would be for alternas, and darn convenient for us to stop and look.
Correct. A cut is a roadcut where part of a hill is removed to allow a road to pass. These porous openings to the underground crevices are often hunted by alterna (and many other snakes) at night.
Here is a cut in Crockett county that has produced alterna -

The only time you'd find them AOR is in the spring, just before or right after a rain.
In east Texas I have found many speckled kings AOR after a nice spring shower, but also AOR at night in the summer months.
Is trash a viable habitat for alternas in the eastern (greener, more populated) parts of its range?
No. A few alterna have been found under rocks, but many people have flipped thousands of rocks over the years and failed to find them. They are generally underground or out foraging at night.
My gut is telling me north of Dryden and La Linda. I hear the more north of Devil's river you can get, the better - but it is challenging unless you have a 4x4. I know no one is going to give away their hotspot, but any pointers for location would be great.
More alterna are found along the big roadcuts along highway 90 near Langtry or on 277 north of Del Rio (around Loma Alta). It appears they are slightly more common there and if your goal is to find one, that is a good place to start looking.
If Davis Mountain produces so few specimens, why is it such a favorite with a lot of people?
Generally rarer localities make the snakes more "popular". I personally don't find the Davis Mtn. snakes all that pretty, but to each their own.
What are the challenges with collecting south of the border?
Simple. It is illegal to collect wildlife in Mexico without the appropriate scientific collecting permits. Those are not given out very readily, only to credentialed scientist doing documented research, and certainly not just for collecting snakes.
Furthermore, if you have ever travelled in Mexico, one of the rarest things in that country is a quiet road to roadhunt. There are some, but most of the paved roads are very busy.
I don't mind spending mordida to keep what I catch
Offering mordida is a good way to end up in prison. The 1970s are over and Mexico has cracked down on corruption tremendously. If you have that little respect for the nation, its people, and its laws you should probably stay home.
Furthermore, even if you were irresponsible enough to catch a snake in Mexico, do you plan to offer mordita to the US Customs guys as well? It is of course illegal to bring live animals or animal products into the US from Mexico. The fines start at around $10,000.
You can pretty much forget the idea of collecting there, and frankly, alterna is not likely to be any more common on the south side of the river.
I am not into hot stuff, but is really true that Mojaves are also in this range?
Mohaves get into the far western counties of Texas (check a range map). Where Mojaves are common, alterna are few and far in between.
Remember, alterna hunting isn't like catching speckled kingsnakes.
Even if you go out under ideal conditions, you are likely not to find one. If you spend a week hunting all night every night in the good time of the year (May-July), you have a better chance of finding one. Some people get lucky and find one 10 minutes out on their first night. Many other people spend years looking to find their first.
Not trying to discourage you, just make sure you have realistic expectations.
-----
Chris Harrison
San Antonio, TX