Here's a simple theory that I think accommodates the evidence as we know it:
EN and his Audi-driving friend are the drug bigwigs in the neighborhood. BM and KM have a beef with them - think they got ripped off in an earlier drug deal, maybe, or fighting over turf.
They set out in the green car, with gun, to go to the park/school to find EN and hash it out with him. EN spots them before they spot him, and he calls the Audi driver. Audi driver comes, picks up EN, BM sees EN get into the Audi and realizes EN is about to get away. He shouts something angrily, waves the gun out the window or points it at them, and the chase is on.
After they've been chased for a bit, EN says "*advertiser censored** this. I've got a gun. Stop here and I'll show them what's what." Audi stops, fires at Buick. BM panics, he didn't realize EN had his gun with him. Buick flees. Audi follows. Audi gets to cul de sac, I dunno who shot first but they're both still pissed and amped up from the chase and they both open fire. TM had run outside when she heard the Buick squeal to a stop, and got caught in the crossover. The end.
Of course, they realize they can't tell the police what happened, so the lies start immediately.
They protect EN as long as they could, and they're still protecting the Audi driver, because snitches get stitches, doncha know.