Could the blue marks that appeared on the face, hands, and back of the deceased, as well as his thrashing and calling out the name of another deceased person during his sleep on the night of his death, be among the signs of a bad end mentioned in the Holy Quran, or are they a result of natural causes such as circulatory disorders associated with old age?

You must think well of your father, as long as he was regular in prayer in his last years and was beloved by people, for "You are Allah’s witnesses on earth." What the questioner saw on his father’s body cannot be interpreted in light of the verses pertaining to disbelievers, for the father was a Muslim who prayed. The important thing is to pray for mercy for your father, seek forgiveness for him, give charity on his behalf, and perform Hajj if possible.