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.
Kenny's family doesn't seem to like him much. What's more surprising is why David put up with him at all.
David is not an extremely social man, he gets along with people well enough to sell his pot very well, but probably doesn't like being around people a whole lot. In the beginning of the movie, aside from selling pot, the only things David seems to do on a common basis are go to Rose's strip club and the bar where he was abducted by Brad's thugs. David likely sees Kenny as a nice kid and an outcast kindred spirit and tries to help him out some and maybe give him some guidance; since they live in the same apartment building, Kenny and David seem to know some stuff about each other's home lives and David probably sees that Kenny doesn't have an extremely happy home life.
Why would Kenny be on antibiotics for an allergic reaction to a venom? That doesn't make any sense.
Probably just a mistake on the writer's part. Just apply MST3K Mantra here.
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.
Most likely, David turned from Jerk Ass to Jerk with a Heart of Gold. He started to bond with them, and his conscience probably started to gnaw at him not to abandon them. Hence why he not only goes back, but offers to split the payment with them equally. In the end it turns out Brad had no intention of paying him, so David in turn has no problem turning him into Don and the DEA.
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.
They probably figured the less Don's family got involved with them, the better. It was probably a good thing they failed though, since Pablo Chacon was an Implacable Man and very Dangerously Genre Savvy, and would've killed them near the end of the film had Don not of been there to save 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.