Are not the two hadiths that state that the Black Stone removes sins contradictory to the concept that the removal of sins is a benefit? And how can they be reconciled, and is the belief that the Stone removes sins considered shirk (polytheism)?

Kissing the Black Stone is an act of compliance with what the Prophet, peace and blessings be upon him, did, not out of a belief in the Stone's ability to benefit or harm. The Stone does not forgive sins; Allah alone is the one who forgives them. Wiping the Black Stone is a means for expiating sins by Allah's decree, similar to ablution and prayer. It is an act of obedience and compliance with the Sharia (definition), not a belief in the Stone's inherent power.