Are the attributes shared between God and His servants—such as generosity and mercy—considered partnership with God, or is there no partner to God in anything, and has God sent down any authority for this type of polytheism?

This is a sharing in wording and mental meaning, not in reality and existence. For example, mercy exists in humans and animals, while each retains its distinctiveness. So, what about the difference between the Creator and the created?! Allah is One in His essence and attributes; He has no partner, as the Almighty said: "And there was not to Him any partner in [His] sovereignty." Therefore, agreement in name does not necessitate resemblance between the Creator and the created.