What is the interpretation of the physical changes that occurred to my deceased mother during ghusl, such as smiling, changes in skin color, and the disappearance of signs of fatigue? Are these signs of a good ending, even though she uttered words contrary to creed before her death? And what should I do for her after her death?

We ask God to have mercy on the mother, and to accept her acts of worship and good deeds. She is not to be held accountable for words whose meaning she did not know. The color you noticed on her back may be due to blood pooling or other reasons related to the effects of the illness. One should not think ill of her; rather, it is imperative to think well of her, conceal her faults, seek forgiveness for her, give charity on her behalf, and perform Hajj and Umrah (definition) if possible.