What about how Lake Superior is shaped indicates anything weird is going on?

I was also going to mention that.

Also, it displays the same equiangular vertex of three rifts that we see with the Arabian, Nubian and Somalian plates. Which you also see in oxide scum on molten lead. Or on oatmeal, just before it's ready.

Non-mobile link https://en.wikipedia.org/wiki/Midcontinent_Rift_System

Does this also explain the extended butte-and-valley terrain in the Southwest?

The butte-and-valley terrain, as you called it, is more from the uplift and slow erosion of a large area called the Colorado Plateau. The various sediments deposited on and off over a billion years or so have different erosion rates. When a thick, more weatherproof, sandstone overlies a more erodable shale you get those big cliffs and buttes.

The area covered by Nevada, on the other hand, is being pulled apart over the whole state. This creates what is called a basin and range topography. The extension is mostly east-west and you can see the ranges and valleys mostly are north-south due to this. Lots of geothermal there as the spreading rock allows hot rock from below to move close to the surface.