The MC has a pretty bad memory, that's true. And while I would agree that its kind of weird for him not to remember his relationship with characters such as Viktoria or Stephanie, I don't think totally unrealistic. If someone spends decades alone and wallowing in his own thoughts I definitely can see him forgetting and misremembering quite a lot of stuff.
The main character "has a pretty bad memory", and in the same time he is a teenager genius who has plans to build the company, have a lot of money, change his past, save his family, all alone, in the body of 9-10 years old boy. Yes, very "realistic". I believe you.
I can remind just about his best results in math, speak about the exam in math, where he very easy and playfully has the best marks and results. With a "bad" memory. Again.
In one moment he has no memory, when it is comfortable for the story, in another he is pretty smart, active, collect all women (sounds too well for reality and not for a dream), has no mistakes and is lucky in every step. Yes, I totally "believe" in your words and arguments.