If Casey's defense team uses SODDI as their case-in-chief, an accident is still on the table and will almost assuredly be considered and discussed, because the evidence supporting the State's murder one charge is extremely weak, to say the least.
Allegedly, planning a murder and then driving around with the victim's body in the trunk of their car for days, as the State contends, is not an act that supports a planned murder. Allegedly, disposing of the body by leaving it on the surface of the ground close to the road and near the home, as the State contends, is not act that supports a planned murder. Both of these items of evidence work strongly against murder one and strongly for reasonable doubt, yet it's the State that will introduce this evidence at trial.
When the defense presents its case-in-chief, it would not even have to bring these points up, because these items of evidence will be presented by the prosecutors. And it's the State that carries the full burden of proof, yet reasonable doubt is naturally embedded and self-highlighting within the evidence they must necessarily claim constitutes a planned murder.
Net, the prosecutor's own evidence easily defeats their charge of Casey committing a premeditated murder.