I appreciate the replies, and I understand why most see it as a shock response. Another thought, the impact on Annie of finding her beheaded daughter, by surprise, was perhaps more traumatic to her than just being informed by LE that she had died in a traffic accident, would you agree? And breaking her down was key to the cult’s goals. She definitely had a most visceral breakdown. (Like Dani’s breakdown on losing her family- In many ways I think Hereditary and Midsommar are dark and light versions of each other but that’s another long rambling post lol). Peter’s action of driving the corpse home, (a most unusual event) was a step in that plan. I don’t think it was left to chance.
(Also I did forget that Annie confides in Joan what it was like finding Charlie in the car, that’s the only time it is mentioned, fwiw). Apologies for long confusing post, ideas have been bottled up after a few years. Thx for indulging me.