I think this was a very suspicious death and the autopsy would have been extremely thorough.
And, anyway, who would really want him dead? He seems to have been extremely loyal and supportive of LV. It seems his new wife had been someone she knew, ie he was introduced to her by his sister and maybe CD. There was gossip about LV and AC having a fight in the street, but it appears he was seen frequently on video visiting the storage locker, so they were still connected.
Pulmonary Edema is linked to emotional stress, and I would think AC was under a lot of stress - he'd shoot at/kill people, was under investigation for homicide, newly married, and certainly knew about the missing children if not directly involved.