What is the ruling on saying: "Pardon us, Messenger of Allah" or "Forgive us, O Messenger of Allah" with the intention of lamentation and remorse? And is this considered shirk (polytheism)?

Using rhetorical expressions like "May my father and mother be sacrificed for you, O Messenger of Allah" to express grief and sorrow or to disavow an attack on the Prophet, peace and blessings be upon him, is not polytheism (shirk (definition)). Some Companions addressed the Prophet after his death, not in a manner of seeking something from him, as Fatima, may Allah be pleased with her, did when she said: "O my father! He answered a Lord who called him." And as Abu Bakr kissed the Prophet after his death and said: "May my father and mother be sacrificed for you; you were good in life and in death." This invocation is not intended literally, but rather to express lamentation and pain, or to bring the invoked one to mind and address the witnesses with the heart, as mentioned by Ibn Taymiyyah. However, these phrases should be avoided in front of those who do not understand their intended meaning, to prevent misunderstanding.