Is prostrating to a person, even in jest and as a greeting, considered polytheism (shirk) or a major sin? And is repentance for it accepted, based on His (Allah's) saying: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills"?

Prostrating to other than Allah without intending worship is a sin, not polytheism (shirk). The one who repents from a sin is like one who has no sin. Allah forgives whoever repents, even if they committed an act of disbelief, as He Almighty says: "Say to those who have disbelieved, if they cease, what has passed will be forgiven for them." As for the verse mentioned in the question, it pertains to those who died polytheists and unrepentant.