Is a son's kissing of his mother's hand, or his father's head or feet, considered polytheism (shirk)? And is there any evidence from the Sunnah for its permissibility?

It is permissible to kiss the hands and feet of one's parents as a sign of honor and filial piety towards them. The Sharia does not prohibit this, nor is it considered polytheism (shirk). As for the saying that "doubt is half of knowledge," it is false and baseless, because doubt is blameworthy. The claim by some theologians that doubt is the first obligation to attain certainty is a rejected statement by the righteous predecessors (Salaf al-Salih) and scholars of knowledge such as Ibn Taymiyyah, Ibn al-Qayyim, Ibn Hajar, and Al-Qurtubi.