Ok, so now I am a crypto exchange, which credited peoples' accounts based on expected safe finalization (which arguably doesn't exist: that's the core issue her

Ok, so now I am a crypto exchange, which credited peoples' accounts based on expected safe finalization (which arguably doesn't exist: that's the core issue here... these kinds of services are arguably impossible); and your hard fork is effectively happening in the past which is unlike any normal hard fork (which is always scheduled in the future). The term "roll back" absolutely applies here. FWIW, I expect most exchanges to grind to a complete halt upon noticing this much chain reorganization (enough to break their finalization assumptions) and so the entire ecosystem would have to sit around and figure out what to do manually, with different participants having noticed at different moments and with different losses due to already-spent debts... it would be a complete mess to repair.