I know a bit about head injury for someone non-medical. During uni I worked one summer at a rehab home for people with head injuries, and earlier this year one of my brothers got a severe head injury. Luckily he's recovering with his personality intact, but while he was in a coma we read a lot about head injuries.

Basically, large personality changes are very common with a severe head injury (which means in a coma for over a day, which Louisa was, yes, she opened her eyes once, but that wouldn't meet the modern definition of out-of-a-coma).

She would also almost certainly be suffering from unrecognised fatigue (one weird thing, we had to teach my brother how to recognise when he was tired, I never really thought of it before but one of the things your brain does is monitors how your brain is feeling, and that can be disrupted too). A big revolution in treatment of head injuried patients in the 1990s was recognising what a major issue fatigue is and how much rest they need (absolute quiet rest). If this wasn't recognised in the 1980s, then I doubt it was recognised in the 1810s. Benwick sitting quietly with her is actually doing a very good thing for her recovery, if he could just stop reading poetry to her or whispering gently that would be better.