We can’t test for the vast majority of things that we CAN conceive of. How could you disprove that ghosts don’t exist in an invisible, undetectable parallel dimension to our own?

Why would you need to? If they’re undetectable, nobody could have ever seen them. Saying that ghosts exists in an undetectable parallel dimension is effectively the same as saying that they don’t exist at all and every sighting claim is bullshit.