The time of death will be recorded as the moment when resucitation attempts were abandoned, but the real time of death can be determined by the autopsy, if there was lividity when EMT arrived then he was long gone.
You can be brain dead and still have a beating heart as we know from people who are on life support, the lack of oxygen when he stopped breathing would have caused brain death very quickly, so he may have had a faint pulse when Murrey found him but be brain dead.
Whether or not he had Prince present when he was pretending to try and resucitate Michael is legally irrelavent, I would imagine it is only about what he gave him, what he had in terms of monitoring him, and how he acted to deal with the emergency. Sounds to me like he 'phoned a friend, and maybe a lawyer before he called 911.
Prince himself will be the one to say what happened, and La Toya would have got it from him, so to say she was not at the scene so didn't know is pathetic, and shows the lack of defence Murrey has. I hope La Toya checks her facts before she makes statements, it doesn't help if it turns out she didn't.