Among the theories about the incident in Marabar Caves involving Adela- Adela hallucinated, the guide is the culprit, or a stranger did it

I think this is up to the reader. Although the book describes India as a "muddle", there is also a sense of mystery. The Marabar Caves represent all that is alien about nature. The caves are older than anything else on the earth and embody nothingness and emptines: a literal void in the earth.