An exchange uses a database to store everyones information, such as the amount of BTC you have. Say the exchange has 1000 BTC total from a spread of 100 users. Someone hacks the database (or the site/owner changes the database) and adds a user and sets his BTC amount to 1000 BTC -- even though those BTC don't actually exist.
This user can now proceed to sell, buy, cashout ect. -- All without being noticed as long as more and more users use the site. Now, if one day the user base becomes too low or everyone decides they need to cash out -- IT WILL NOT BE POSSIBLE.
A system in place needs to be acted upon on all the exchanges ASAP. I call upon them to do this for the safely of BTC and its users.
This is trivially done. Check the total of BTC in the DB and compare it to the exchanges BTC wallet(s). Perhaps before each withdrawal.
It's most likely the exchanges have many such checks already in their code.
Are you suggesting the exchange should operate directly by depositing BTC as trades occur? I guess you can't be - because that would be utterly impractical due to the slowness - and would mean that if things were hacked/glitched - there'd be no repair possible via rollback.
(reverted trades do occur on standard exchanges - happened on NASDAQ earlier this year due to glitch apparently)
I'm not clear on how the addresses you talk about allow us to verify things are legit - to me it sounds unlikely to be useful. Please give an example of a previously empty exchange followed by a single trade between two users, and what is where in your scenario.