Ohtani has been playing stateside for SEVEN years now - it's not like he's still kicking off the jet lag and waiting for the rest of his things to arrive in the mail to unpack like it's spring 2018 and doesn't know anyone else. I can see needing some assistance initially getting settled in the US from a trusted friend and assistant you've known 5-6 years prior to that point, but eventually when you realize the dollars flying in the door probably exceed the financial expertise of your interpreter you'd think it might be a good idea to set things up a bit more ironclad than what is being made out to have been the case. That is, unless the story isn't being completely truthful.
To me, it's still alot more believable that Mizuhara was Ohatani's middle man when it came to making bets on the scope and scale that appeared to have occurred, and they were smart enough to not text each other about it largely because they're practically inseparable and could just as easily talk about making bets, money transfers, etc. using the account Mizuhara had ready access to. Once they realized the entity/bookie they were using was under investigation by the Feds, suddenly it was time to craft this story. Ohtani simply doesn't come off to me as a doe-eyed, gullible, victim who's clueless to what's going on around him financially.