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How and when do investors who lost money during this FTX bankruptcy get their balance back in on their account with the exchange?
Most likely they will never do. Even after FTX reopen. Its possible that FTX CEO will "find" as much as it is possible to pay US users to not be chased by US authorities not carrying about others. That wat chapter 11 is - bancrupcy.
There is also while Chapter 13 - a reorganization. After this, as long as I know, you can have a fresh start without paying all your liabilities. You only have to have reorganization plan and the bankruptcy court's need to accept it even without the acceptance of creditors. SBF have frieds to help with that.