You've got to understand, training is one thing, but acting within the bounds of the law is sometimes something else altogether. Police could know in their gut, 100% without question that a party is in a dangerous situation, but if the laws of the state do not permit them to do anything meaningful, then the training then becomes unimpactful. Keep in mind, they're job is to enforce the laws, not serve as marital counselors.
The training for police should make them aware of the dangers of DV situations, the threats either party is facing, and doing what they can to separate and/or diffuse the conflict.
Real meaningful change is going to come through the laws. I'm not a big fan of just trying to separate them for the night. Personally, I think it should be legislated, that if police are called for a DV situation, there should be a mandatory, 72 hour temporary restraining order on both parties. Keeping them separate and giving parties time to settle and think logically about the situation might lead to fewer people going right back into toxic situations.