Was it ever explained how Brad knew Pablo and why he used his name as an alias for picking up the weed?
This troper imagines that Pablo is well-known in the trafficking world but not enough for someone who's a small-time dealer to recognize the name. So Brad can drop it as an alias without David knowing the significance, though to dedicated cartel members it obviously means something.
Where is Kenny's family?
Like Kenny said in the beginning, his mother left for a drink with her friend... 2 weeks ago.
Why would Kenny be on antibiotics for an allergic reaction to a venom? That doesn't make any sense.
What would be the harm in leaving his surrogate family in a carnival in Texas (?) while he goes and delivers the drugs? He needed them to get across the border and they're in America, so they can likely take public transportation back to Denver. I suppose if he left them with no money that would be a problem, but he could have just given them money for a bus ticket back to Denver.
Similarly, what's the harm in leaving them with Kenny while he delivers the pot? This was, quite literally, a life-or-death situation for David.
What's the point in stealing the keys to Don's RV to switch out the fuel hoses? Is waiting until the next morning that awful? And best case scenario, they have a DEA agent who knows they sabotaged their vehicle to fix theirs, which just... isn't good for them.
What happened to Casey that she was willing to go all the way to Mexico with people she has known for less than a moment?
$1,000 is a pretty good amount of money for her and she lives a risky lifestyle anyways sleeping on the streets and such. Casey has probably trusted herself in the company of much stranger people than David and Kenny.
The trip also offered her a roof over her head for at least a day or two and the promise of at least getting some half-decent food along the way, which is probably a lot more than she usually gets.