The welfare check WAS a setup. It was a "clean up" call, and likely used by the cartel as a propaganda tool. Simply, they were done with that location, and maximized the affect. Usually by that time, the neighbors are complaining about the odor.
Life is tough, but it's tougher when you're stupid.

the welfare check
If you listen they saw the Welfare Check sitting in dispatch queue and self-assigned the call saying: if we take some of these run-of-the-mill calls that usually turn out to be nothing then maybe Dispatch will be more likely to give us more of the exciting calls.
Not a setup, but luck of the draw (and writers ex machina).

excellent, umetoo, thanks!
"I offered him a lateral move to Austin." "That's like a duck making a lateral move to a l'Orange!"

The video is just a perspective, showing the cartel placing a hit on the cops. It's not to be taken as 100% correct or realistic.
Those ghetto Mexican gangbangers, including the gross-out Mexican chola chick just belong to a gang, and their gang is probably connected with the cartel. So sometimes they get orders passed down to them, to whack people.
Idk about the welfare check thing