One thing that I've been wondering about for the other black hole and am now wondering again for ours: Why is it possible to image the polarization with such an improved resolution compared to the "base"? - You see pretty fine lines compared to the very blobby smudge of the black hole.

That's because the lines indicating the polarisation are drawn manually on top of the picture. The polarisation is just a direction everywhere, so they have drawn lines that follow that direction. I would have preferred a lot if they had used a different color for those to avoid that confusion.