I also thought the middle picture looks weird! There's a bunch of things that look weird to me about it but I can explain them away:
-There is a lot of track that is up in the air but there are no strut supports shown, so there is just a ton of track suspended in the sky! However these suspended track pieces seem to be an elevated curve and a spiral, and in the final game, neither of these track pieces have struts under them, so they would also appear suspended in the air.
-The grass is a similar light green to that used on the top of hills in the final game and the horizon almost seems to hit the grass before the elevated track pieces appear, suggesting that the elevated pieces are placed on lower ground near the hill. This makes it seem like the player car is on top of a hill and somehow the elevated turn and spiral are even higher than the hill without themselves resting on it (it feels like they are suspended twice as high as they should be). However, I think the light green grass in this screenshot is just a color difference from taking the picture with a camera, and there is nothing to conclusively say that the player car is on top of a hill. I also think it is just camera angle and perspective that makes the elevated curve and spiral seem to be at a higher than normal elevation.
-It kind of looks like the elevated turn and the spiral are placed at a 45 degree angle to where the player car is, which wouldn't be possible with the track editor the final game has. However, I think the player car is on one of those flat ground track pieces that takes up 4 terrain blocks and briefly goes at a 45 degree angle.
Regarding the boxy walls on the jumps in the proto screenshot, maybe the added polygons needed to do the boxes dipped performance too much so it was just an optimization choice to make them paper thin. On the other hand, that's probably not right, as when you place a straight track on the edge of a hill (so it is inclined), it has boxy guardrails on it. I always thought it was interesting how you can drive right through those boxy guardrails without crashing, while hitting every other track piece that has a guardrail or wall will result in a crash.