Stats are correct in this case. The amount of triangles does not change.
Setting material to double-sided increases strain on rasterizer and situationally, on pixel shader and you won’t see any performance difference, until these become the bottleneck.
Depends on circumstances, but occasionally you might want to save on one extra draw call. Triangles, that are not visible, are not getting culled. Not to full extent you’d expect.
Doing so is debatable, as viewed in given context, is definitely not a ground truth and is a subject to discussion. No doubt, there are cases when you’d want to make exactly opposite. But commonly, there is a trunkload of other reasons to split them in two materials. Typically you’d need another shading model and some vertex animation for the flag, which is not needed for the pole.
Bottomline is, common sense should be applied first of all, when deciding things like that, for it is quite hard to anticipate the performance hit of such choices.