He didn't commingle accounts. He stole from customers to keep Alameda afloat. This is why the money never got converted into actual crypto, and why there are no assets: the money got used, by Alameda, in what must have been some truly cataclysmic trading.
It also sounds like alameda's job was to market make the FTX coin, which they presumably did with that money in part. And likely kept doing until they blew up.
He's a fraud. He knows he's a fraud, and his PR campaign is an attempt to prove he's not a fraud but instead just really really dumb and stuff. Because being dumb isn't illegal, but running a fraud is.
Well, the fact that he's been waging a PR campaign lately is, in itself, pretty damning proof that he's also really really dumb, in addition to being a fraud. That his lawyers haven't duct taped his ass to a chair, smashed his phone and computers, burned any paper he could use to write a letter to a random reporter, and cut the power lines to his house is honestly surprising at this point. They must have explained why he needs to stop talking and stay quiet multiple times at this point; nobody talks their way out of being charged by the feds. And yet, here we are.
Everything he's done since FTX first crashed has pretty much been straight out of the most fantastical dream any federal prosecutor has woken up from in the middle of the night, certain that no defendant on the planet would actually be that dumb.
My best guess is that a small part of the billions went towards mansions in the Bahamas, another small part went towards political bribes, and a rather large part went to various counterparties that traded against Alameda.
It's pretty easy to lose any amount of money if you're paying $10 for $5 bills.
It also sounds like alameda's job was to market make the FTX coin, which they presumably did with that money in part. And likely kept doing until they blew up.
He's a fraud. He knows he's a fraud, and his PR campaign is an attempt to prove he's not a fraud but instead just really really dumb and stuff. Because being dumb isn't illegal, but running a fraud is.