So, this part will be long, but to get basic information & types of TBIs out of the way;

A Traumatic Brain Injury — Better known as a TBI — often occurs when a person receives a severe blow to the head, and ranges from a mild concussion to severe brain damage that can lead to death.

Theres two types of TBIS; Penetrating & Non-Penetrating. A Penetrative TBI occurs when an object pierces the skull & enters the brain tissue, and a Non-Penetrative TBI occurs when an outside force is strong enough for the brain to move within the skull.

A DAI is one of the most common types of TBI; mostly occurs in sports injuries, falls, or car accidents.

A Concussion is a Mild Form of TBI and takes anywhere from days to months to heal.

A Hematoma is bleeding in/around the brain caused by a burst blood vessel.

Contusions are a bruising or swelling of the brain that happens when very small blood vessels bleed into the brain tissue, caused by the brain bouncing back and forth within the skull. Usually occurs in high-speed car accidents or shaken baby syndrome.

A Skull Fracture is the result of blunt force trauma and can damage the membranes / blood vessels

CTE ; Most Commonly Occurs in sports with repeated small head impacts & research suggests that repeated TBIs cause it.

Post-Traumatic Dementia (PTD) occurs after a single, severe TBI & may be associated with increased dementia risk later in life.

NOW

To talk about Graces TBIs specifically; it's really important to note that realistically, he without a doubt would've gotten Second-Impact Syndrome, but we're going to disregard that because it is almost always fatal and, well, we see he obviously does not die from the injuries.

However, he does still suffer two severe head injuries during the Time Go Fishing scene; One of which caused momentary unconciousness on impact, and the other occured under several G's of force, and was followed up by him being put into unconciousness by Armando shortly after.

Trying to stay canon to what occurs in the movie/book & Graces ability to function afterwards, Grace probably suffered from at the very minimum a very severe concussion, which likely would've left him with symptoms for months after the injuries & still likely would've had atleast some mild permanent affects on certain functions. He more likely suffered from either a DAI or a Contusion, & rehabilitation / physical/mental therapy would've been required/advised for him to return to "as normal" as you can be after a head injury like that.

Technically, there probably would've been motor function issues, headaches/migraines, amnesia, mood changes/swings (depression or otherwise), fatigue & potentially even Seizures.

There is a Very High Chance that Grace would be considered disabled after those head injuries due to long term / permanent affects on the brain. And with the pre-existing affects of the Amnesia Drug with the Head Injuries, Grace probably does get Dementia later in life. Balance Issues, Memory Issues & Speech/Vision/Hearing Issues developing after even severe concussions are common.

This is just something to keep in mind to all my fic writers. If ANY of this information is inaccurate or you know something else thats important, feel free to add onto this post.