Tesla replaced the visor "under warranty."
I saw how to remove it: The U-shaped trim piece pops up. Use a plastic prybar tool at each side of the "U." Then the visor is secured with a single Torq screw. (It has a "lip" to hold the other side. (Not your traditional 2-screw mount!)
But even with the visor removed, I could not manipulate the residual part so as to fit it to the mirror cover and glue it. The design appears to be faulty: too much stress on a tiny area + normal plastic deterioration with age = faulty design.
I had a BMW many of whose plastic parts deteriorated with age, even where not(!) exposed to UV!
I had a chemist friend who worked for Bell Labs, "back in the day," and it was a major challenge to formulate plastic parts to withstand stress and age. ABS was found to be much "tougher" and more durable than PVC. (Remember the old "500" telephone?)