It might be that soaking the buttons in a 50/50 mix of acetone and transmission fluid for a day or two will free the one up? A penetrating oil might work too. I prefer to use the 50/50 mix, as I’ve had better results in freeing stubborn fasteners with it. I didn’t have to remove the buttons on my restoration, but did have to remove the regulator assembly to replace the window crank springs. In doing that the buttons, still riveted to the “arms” were dressed with a file, cleaned and greased. At least one of the buttons needed a small, slightly flattened segment reshaped to a curve.