How do we explain the return of some patients in good health after their visits to shrines and tombs, and does this mean that it is permissible to go to them as a last resort?

Lawful visits to graves are permissible, whereas visits intended for seeking intercession through the deceased, or supplicating Allah at the grave, or praying there, are considered innovations (bid‘ah). As for supplicating the deceased, or seeking their aid, or asking them to fulfill needs, this is a major shirk that Allah does not forgive except through repentance.

Any healing that some people may experience during these prohibited visits is a trial and test from Allah. Sheikh Ibn Baz clarified that such healing might be a gradual enticement from Satan to lead a person into shirk, or a mere coincidence of divine destiny, or a test from Allah to distinguish the evil from the good. The Healer is Allah alone, and the deceased possesses no power to harm or benefit themselves. Healing is not achieved by going to graves, but rather through monotheism, supplicating Allah, and seeking treatment through permissible means.