I have a theory, and I know it's probably not what the creator intended, but I thought it was worth mentioning. Expect some rambling.
What if Madotsuki is not a Japanese girl? What if she is, for instance, an American? If she couldn't speak Japanese it would explain her inability to dream up more than sounds and numbers.
And, on top of that, it would make the trauma theories involving other characters that much more heavy. Imagine if the only people you were comfortable talking to, let alone even ABLE to talk to, were suddenly gone from your life? For instance, if Monoko got in an accident, if Monoe simply moved too far away to maintain a relationship, if Seccom overdosed ("crashed"), if Poniko was drowned, etc. Although any theories, not just these, involving these characters are equally likely in this scenario. Also, I think Poniko is also a definite non-Japanese character. Ironically, Madotsuki and Poniko are the only characters who have a "normal" room. Poniko is not black and white, which could mean she is a fresher memory, or perhaps Madotsuki connected with her the most, being both foreigners.
Either way, these characters cannot speak to her anymore and that is traumatic. For whatever reason, Madotsuki is unwilling to find new people that she can communicate with.
Madotsuki could be a given name from her friends, i.e. Madotsuki's current personality is "like talking to a wall" except when it comes to her friends, where she is an "open window". The only one left to open up her mind is herself.
Culture shock could explain some of the environments she finds herself in. What if she's from an American city, and therefore isn't used to open spaces, explaining the unsettling lonely areas, but she's not used to the Japanese cities either because of culture shock? No matter where she goes it is uncomfortable.
The only question is, who is supporting her Hikikomori state? I think she might be supported by a boyfriend or husband who is stationed in Japan. My mother actually was married to a soldier who had to live in Okinawa, and her description reminds me of Madotsuki's situation. She had a tiny apartment, in an alien environment, with no one to talk to, and her only companion was separated from her for extended periods of time. This would place her in a particularly sensitive post-war environment of Japan, where military installations of US troops continues to this day.
In between the short visits from her companion, she established relationships with the only English speakers she could. When they died or left, she had absolutely nothing to do. The deaths of her friends (if they were brutal enough) in addition to whatever odd culture elements frighten her, would keep her from leaving her room. All the stress would explain what she does at the end of the game. I know my mom was close to a breaking point after living on her own for a while.