The best person to ask about Type 2 diabetes is your doctor (or the doctor of the person with diabetes).
It is true that a person can be diagnosed with Type 2 diabetes and eventually be able to manage his blood sugar levels by sticking with the proper diet for diabetics, losing weight, and eating properly from then on.
I know two people who have done that and who were able to stop taking insulin and manage their condition just by eating properly.
When a person's blood sugar levels are too high, though, damage can occur in a number of ways, and once some damage has occurred it cannot be reversed (so if the person diagnosed with diabetes manages to be able to later control his blood sugar levels without medication it doesn't mean he can avoid/undo any damage that already took place while his blood sugar was high.
Infection can cause a rise in blood sugar, whether or not a person has Type 2 diabetes. So can stress.
With regard to injury, I'm not knowledgeable enough to address the question of any link between head injury and elevated blood sugar.
Here are some links related to "post traumatic hyperglycemia". The first one suggests that there can be a connection between elevated blood sugar levels and head trauma. It seems, however, that this may be controversial.
The second one is additional information:
Even without knowledge about rises in blood sugar associated with head trauma, I do know that all incidents of hitting one's head don't result in the same degree of trauma. Generally, and to the best of my knowledge, the measure of severity of injury to the brain is whether someone loses consciousness at all, and if he does how long he remains unconscious. The longer someone remains unconscious, the more serious the injury.
In other words, if a person falls, hits his head, but doesn't lose consciousness he doesn't have to worry as much about trauma as if he lost consciousness briefly; and the person who lost consciousness longer needs to be more concerned than if he was "out" for more than a brief moment or two.
If you wanted to research what's "out there" with regard to head trauma and rises in blood sugar levels, you may want to search for a phrase like, "traumatic hyperglycemia" to start your search.