> The cases would still be evaluated as per the state of the law as it stood at the time the questionable acts occurred. If police were operating under QI and QI is later repealed, changing the law's interpretation of their behavior after-the-fact to find them liable is ex post facto law.
No, it's not: ex post facto is a term of art for retroactive criminalization; QI applies only to civil liability. Retrospective enhancements to civil liability are not Constitutionally prohibited.