From all accounts Ohtani doesn't really spend money and lives relatively simply, as far as a superstar athlete in California / Japan can do. I know when he was in Japan, the club he played for basically took care of everything. He just kinda did baseball, almost living in the dorm for the club. I have to believe that basically was what was happening here. The report that just came out makes it clear that this interpreter was more than just a translator for him, serving more or less as his personal assistant, friend and go between. It is pretty crazy that he was able to get that much money without it being noticed, but from what it seems, Ohtani likely just delegated anything not baseball related to this guy. It would be completely believable that he would be the middle man to work on housing, cars, you name it. So, having the access to the money is not completely unsurprising. It happens way more than anyone would like to admit. So many people around athletes take advantage of the situation and often leave them broke. They trust people who they likely shouldn't and they pay for it. I don't know Ohtani's upbringing, but part of the problem with the entire athlete / celebrity bit is many of them just don't come from money. So even if they don't go out and spend it, that money around them attracts people like this.
As far as the bookie, yeah, I'm not completely shocked. They 100% knew the interpreter had access to Ohtani's money and likely looked the other way.