I think Ohtani knew more than he lets on, given how close he was with Ippei. Gambling to the degeneracy extent seems like a hard thing to hide 100%.
But given how Ohtani let his mother control his finances, it's not a stretch that he also let Ippei do the same. Pretty much running all the day-to-day life things for him like how his mother did when he was in Japan. It's kind of unimaginable to ordinary people though, that anyone would be naive enough to do this. In a way you could connect the dots and say his family sheltering him led him to this outcome.
Seems more like a Tim Duncan situation to me, but the IRS/Feds could uncover a lot still. Extradition from Japan to the US for wire fraud adds another dimension. There is a treaty but it won't be as easy as persecuting an American citizen.