If your coin is in the grading stage, does that mean it has already been determined it's genuine and not a fake or an altered coin?
Not necessarily, the website account just shows you where in the process it is. The final grade will obviously be shown later same if it was NGC.
I actually found out the info on the PCGS stage. They actually first determine if it is authentic first during the grading stage
But you won't find out if it's authentic until after the QC stage when the grade or no grade is revealed.