HERE's a detailed timeline of that morning.
IMO, LVD tried to give herself an alibi. She put JJ in the car and then tried to instigate a fight between Alex and Charles. I think her plan was to have Alex shoot Charles
after she left with JJ (which is the story Alex gives to the police).
I think LVD had planned all along to go to a couple public places with video cameras (Walgreens and the fast food drive-through) to set up an alibi that she wasn't anywhere near the house when Charles actually died.
IMO, the complication came when Tylee woke up
before LVD left. I think Alex freaked out and shot Charles before he was supposed to (ie before LVD was gone), and Tylee heard the gunshot, which complicated things, because how do you convince a teenager to lie to the police about what they saw and heard without admitting that the whole thing was a setup?
LVD had had a plan, and when things went sideways, they stuck with it. AC sat around, waiting, pretending that Charles wasn't dead yet to give LVD time to set up an alibi that she was elsewhere, maybe even telling him not to worry about it -- that she'd convince Tylee to lie to the police about when it all happened.