That is a good observation, we didn't see if she was successfully released. It's only implied, and Oda could subvert that expectation if he wished to.
I get the feeling that Brook's ability to influence people's souls through his music to find "courage" will play a part here. Would it be weird if Soul King had a concert mid-fight? He apparently has a personal stake in it.
Not a bad idea at all. I speculated quite a while back something along the lines that you are suggesting. I.e. the ultimate level of the different CoO and how some DF can give almost the same effect.
It is possible that some characters can coat their bodies with AdCoO. Not just prediction but the body seemingly moving on it's own. Think of it as Ultra Instinct. The other use is to "coat their minds" i.e. future sight.
AdCoA can be used to coat the physical body defensively and offensively, even extended to external objects such as weapons.
AdCoC should essentially be the coating of the soul, often described as haki or in some places ryou but they are in fact the same. The soul can be coated for defensive purposes and offensive, i.e. it should be possible to attack someone like King in flame on mode and defeat him 1HKO by attacking his soul rather than this body. These attacks might be the solution to Imu and the Holy Knights.
In my previous prediction I stated some characters like Brook & Big Mom, perhaps even Perona & Moriah, have fruits that might allow the user to attack the opponents soul in a similar way.