TL:DR A brain is turned into mush, destroyed, then 98% of the organ is replaced with other brain tissue. Can that 2% account for memories shared between both brains? Can the brain remember things that happened to the other brain? The new brain is 2% old brain and 98% new brain.
Character A is a self made immortal organism, a girl who learned the magic needed to modify her body both consciously and automatically.
Character A has the ability to modify every cell of her body, 300 years ago she learned bio magic and covered herself with nearly indestructible and regenerating solid plate armor, she has no face and no mouth. Her head looks like an obsidian helmet but is actually made of light sensitive cells which work as eyes. Character A can sometimes grow bio weapons out of her body, like toxic smoke, explosives and flammable acid that burns win contact with water. Most importantly she doesn't age, doesn't get cancer or any disease.
Character A does not have a face or facial muscles, she could build one at any time but for amusement when she is asked what she looks like, she opens her face helmet showing exposed veins, fat tissue and nerves, no bones, she doesn't have bones in her face or skull, just the shell armor.
Character A is living in another more primitive world but comes from our modern world, she has deep knowledge of political and military history, that's why she has worked for kings and emperors as a military commander over the centuries.
Character A was alive for more than three hundred years, but let herself be killed in battle for personal reasons.
The Empire needs Character A alive for their military agenda, Character A is a strong warrior, a good tactician and the best propaganda ever.
Character A has the same importance as Jesus, a living Jesus made of flesh.
The Empire's magicians have learned how to activate the subconscious magic powers that regenerate character A and bring her back to life, After the damage which killed Character A the regeneration takes 2 years to bring her back to life, as compared to the 3 weeks needed to regenerate or modify 45kg of tissue while she was alive.
For my story I need character A to wake up with the mentality of a newborn baby who needs to learn everything from the beginning, a dangerous and lethal newborn baby in the body of an immortal murder machine with centuries worth of muscle memory of killing people.
As character A grows to the mental age of 7, I need some of her old memories to come back, is it possible or do I need to handwave it with something?