Lets say that John has a nice, calm and thoughtful mind. Due to accident, his brain was damaged and now he is angry, unconcentrated and impulsive.
Does his mind still remain nice, calm and thoughtful despite his negative mental states? Or does it changes along with changes to the brain?
If the mind is separate from brain & body, then we have two beings: Mind with positive states and John whose mind is with negative states due to brain damage.
If they are the same, then when brain dies, so does the mind.
Perhaps we could say that the root mind is somehow a more basic thing that is separate, yet operates through the brain to create mental states of anger or loving kindness, concentration or un concentrated mind. The brain stores information, skills and personality. If we say that this subtle mind "jumps" from dead person to person being born, then what is the continuity of development in Buddhist sense if all memory, skills, and personality is wiped out with previous brain? So a person could have developed negative or positive skills, but a newborn baby doesn't have them. This puts difficulty on the idea of practice that spans multiple lives.
As for direct experience of rebirth. Some can say that "I haven't died yet, so I don't know". As it comes to rebirth, there are always TWO points when it occurs. It occurs after death (post-mortem) of this body which we cannot experience now, and it occurs at/before birth of this body (pre-natal). The latter can theoretically be experienced or at least can be remembered. However:
When I (Alex) was born, I didn't come with developed positive or negative skills. I didn't come with knowledge of any languages, and there was nothing from which I could now infer that I inherited my past qualities from previous lives. Nothing to say about the lack of direct experience of rebirth into Alex. It was as if it was my first life...
"Life is a struggle. Life will throw curveballs at you, it will humble you, it will attempt to break you down. And just when you think things are starting to look up, life will smack you back down with ruthless indifference..."