It won't be hard argue it if that is what the final ME report says. If it says the death was a combination of stress from being restrained, intoxication, and pre-existing health conditions, such that all three had to coincide, that is exactly what it means. No matter how extreme the officer's behavior as shown on the video, it's science which determines cause of death.
From a legal standpoint, when you have that kind of situation, it comes down to "foreseeability." The defense will be that they allegedly weren't applying that much pressure and that his death was an unforeseeable freak occurrence, a confluence of what they did with things they were unaware of but which weren't very likely. That's why the cop in the Garner case got off.
I personally think this is where the law of manslaughter needs to change in relation to police officers. Once you have proven the use of excessive force, meaning more force than is necessary to restrain the person given all the circumstances, they ought to be strictly liable for all consequences of the excessive force. If the person is unhurt, then they get lucky. If he's badly hurt, that's bad for them. If he dies, they're fucked. Regardless of foreseeability. Because police are trained in the use of force and ought to know that everyone's body is different and that which may not even harm one person may kill another. So all they should have to prove is 1) excessive force, and 2) resulting harm, not foreseeable resulting harm.
All that really matters is the excessive use of force. If the force applied was reasonable under the circumstances, they're not going to be at fault even if the harm was foreseeable, right? So use reasonable force and there's nothing to worry about. Use excessive force, and face whatever consequences ensue.
This is the only way to really deter the use of excessive force.
Unfortunately, that isn't the law and I'm getting little worried about how that ME report will affect the outcome.