Would what I uttered in words, intending to describe my grandmother's bad fate due to her actions, be considered presuming upon God, nullifying good deeds? And will my repentance be accepted if I repent?

Your statement about your grandmother, "By God, she will see her fate," is considered speaking about Allah without knowledge and assurance, and severe warnings have been mentioned regarding that. The obligation is to abundantly seek forgiveness and repent, while also emphasizing the prohibition of mistreating the grandmother, as she has a right to dutifulness and kindness.