You're creating the same false equivalence again by stating that an objective post mortem diagnosis not being practically feasible is the same as an objective cause(s) not existing. In my previous post I linked to multiple pages listing objective causes/mechanisms for two mental illnesses. Diagnosing a mental illness post mortem isn't as simple as finding rhinovirus in a person's nasal cavity, but just because the current knowledge of the disease isn't sufficient to create a satisfactory model that can reliably make a diagnosis post mortem doesn't mean that it can't be done. And if you don't think mental illnesses subside in molecules and physical structures in the brain, where exactly do you think they come from?
Oh, and you kind of can diagnose certain mental illnesses post mortem: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3181616/