What is the ruling on the aforementioned examples that include religious or ethical acts performed partly for worldly reasons (such as reciprocating a favor, or fear of people's كلام [words/gossip], or avoiding punishment)? And what is the precise criterion that makes riya’ (showing off/hypocrisy) a major shirk that expels one from the fold of Islam?

Major riya' (ostentation) is disbelief in the word of Tawhid (definition) (Oneness of God) to conceal one's kufr (disbelief), and it differs from riya' in actions, which is a minor shirk (definition) (polytheism). As for abandoning a sin out of shyness from people, or treating them kindly in return for their good deeds, this is not considered blameworthy nor a form of riya'. One should ignore waswasa (whispers/obsessive thoughts) in this regard.