BBM
I know about the venomous snakes in Arizona and spiders, and anything I don't know I asked my old buddy from kindergarten who went on to get several doctorates in herpetology. I actually put out snake traps for him every summer on our properties in his search for the elusive Arizona Milk Snake. We also swap birds occasionally, I got a smokin' deal trading him 9 Sebastopal geese for a pair of mute swans!
So, I'm looking at this crime as a sudden opportunity, not planned, so pre-planning the dig I haven't factored. To do it quickly, if it was done within 24 hours after the disappearance, 2 people could spend 20-30 minutes and have something sufficient at a depth of 4 feet. That time of the year, the sand is going to be dry for the first feet or so, it gets a bit heavier as you dig deeper. (lightly damp sand vs Dry sand, if you ever had to shovel it). That's also factoring a temp that ranged from 79-106 degrees in a 24 hour time period with partly cloudy skies, low humidity. A solitary person digging would take 45 mins to an hour and that would be early morning, around 3-4:00 am, the temp would be around 82-84, but the lack of sun would help their body from becoming over heated.
When you have flash floods, ANYTHING is possible as far as what is contained in the water. Some of those out buildings could have come several miles up stream. I saw a few "pump houses" close to the dry wash. These are small house that contain the "well head" and "Pressure tanks" when you have a well. There is usually water under the dry wash, so it is common practice to drill your well close to the washes to hit the water table or these under ground rivers below the washes. Some of the buildings could have been chicken or pigeon houses, this is a rural kind of area where the parcels allow for livestock and domestic animals so you'll see small outbuilding all over. Whether they were in use or not, many people in rural areas hold on to things and you never know when you might get more poultry. As far as putting a body in one, the predators down there would have found a way to get in. Bobcats and coyotes can be very industrious when it come to wood, wire and rock barriers. And the vultures and buzzards would have perched on the top of the buildings, to say nothing of the smell.