But the OP is basically talking about IOU's being somehow magically safer on bitshares 2.0, when the reality is they will be subject to exactly the same risk profile as on a centralised exchange.
Thinking more about it .. you are right .. in case of BTC-bitBTC gateways .. but not for bitUSD-USD gateways because they hold their FIAT in a regular BANK!
BTC could be held at Circle, Coinbase, Xapo or some other heavily regulated BTC exchange. All they would have to do is accept bitBTC in exchange for actual BTC which I don't see why they wouldn't because they could immediately exchange for bitUSD on their own exchange.
So for example Coinbase has coinbase USD. They could keep offering coinbase USD or they could offer bitUSD or they could offer a coinbase token on the Bitshares platform itself as a UIA, and then when people buy the coinbase token they can simply send the token to coinbase to get the BTC.
So you could in theory simply buy Coinbase BTC or Coinbase USD on the Bitshares 2 platform itself with Coinbase themselves issuing the UIA. Banks could also issue their own UIA for actual USD on the Bitshares platform so when people want to cash out on the decentralized exchange the trade would be for BitUSD for Santander USD.
So all of this could be done on the decentralized exchange in a decentralized way without much risk. Multisig could be used by Coinbase and by the bank to secure their UIA, and also they have plenty of money to handle that part. The rest would just be a front end on top of Bitshares so for example Crypsy or something similar can convert.
As far as mixing goes, there is no need for that. Confidential transactions would make it so no one knows the amounts you trade for even if they can see all your trades. So this way you're actually safer because you can prove you didn't buy terrorist tokens because your trades are public, but your amounts are private so no one can know what your net worth is.
Total privacy combined with an exchange could bring risks. It also will mean you can't trade actual stocks because there are regulations which have to be followed where you can prove you purchased it. Now of course I could be wrong about the law, or about how it will play out, but I think there is a sort of sweet spot for traders where it's private enough that the world cannot monitor their trades, but transparent enough that regulators and the world can not worry about violation of bank secrecy or other anti-terrorism or anti-money laundering laws.
Ideal situation is you can trade and no one will know how many shares of Apple stock you just bought. They might see you bought some stock, but who cares if they don't know how much? And if you really don't want them to see that you bought the stock, you might be able to use some alias or purchase indirectly, it will have to be worked out to see what regulators are willing to accept. Perhaps we will be stuck using Identabit.