If we're in a simulation, this implies that with high probability either a) the laws of physics in the parent universe are not our own laws of physics (in which case the entire idea of ancestor simulations fails)

It doesn't has to be simulation of ancestor, we may be example of any civilisation, life, etc. While our laws of physics seem complex and weird (for macroscopic effects they generate), they may be actually very primitive in comparison to parent universe physics. We cannot possibly estimate computation power of parent universe computers.