What is the ruling on saying to the deceased, "Remember us with your Lord," or denying their death, or blaming them for departing, or cursing the illness that caused their death, whether verbally or in writing on Facebook, to express grief?

It is forbidden to say "Remember me with your Lord" to the deceased, because it is a request for supplication from the dead. Merely speaking about the deceased using the second-person pronoun without intending to supplicate to them is not polytheism (shirk). Attributing an action to cancer metaphorically is not blameworthy, provided one believes that Allah is the true Doer, though it is better to avoid such phrases. Cursing a disease is not permissible, because it is an objection to Allah's decree, and the one who does so must repent to Allah.