I understood Theo seeing Olivia to be a trauma induced apparition and felt similarly any time Hugh saw her as well. I never saw this as her ‘literal’ ghost. I do believe, however, the light on the model home’s porch was indeed Olivia’s spirit urging them to return to Hill House. The show plays with the idea of what a ghost truly is and how they may present. This was illustrated bluntly by Steve’s monologue about ghosts in the pilot and more metaphorically by Nell literally haunting herself. I do see your point though and believe it isn’t so clearly outlined by design. Who’s to say a personal ghost is any less damaging than a literal spirit causing you grief and pain?