The brightness of the light from your flash falls off with the square of the distance to the subject. For example, if you are using only flash for illumination and something 10 feet away is exposed properly, then something 20 feet away will be 2 f-stops (4x) darker. Something only 100 feet away will be 100 times (almost 7 f-stops) darker.
Conversely, the power of a flash to properly illuminate objects goes up with the square of the distance to them. Camera flashes will have limited power due to needing to be portable, running off of batteries, and not costing a fortune. As a result, camera flashes are only useful for nearby objects, ususally up to a few 10s of feet. For example, using the flash in your point and shoot camera from the stands of a sports stadium to take a picture of something on the field is totally silly. Just because people do it doesn't mean it's a good idea.
Your case is like the sports stadium. Most everything in the picture is so far away that the flash has no effect on it. Regardless of the flash brightness compensation you used, all the light for these pictures was ambient. The flash had nothing to do with any of them, which is why changing its brightness a few f-stops either way didn't matter.
You can see that the flash had no effect by looking at the foreground at the bottom of the picture. If the flash had any visible effect, you'd find it there. The foreground would look brighter than distant objects. This is not the case, so the light from the flash was already so dim as to not matter at the distance of the foreground.
The reason the flash pictures look different from the last picture taken without a flash is how your camera handles color balance. Flash is "white" light, fairly close to sunlight in color. However, most artificial lights are considerably more yellow and orange. When using a flash, the camera assumed the scene would be lit by the flash and applied color compenstation accordingly. However, your scene was in reality lit by yellow artificial light, so you can see the obvious yellow cast. For the bottom picture, your camera apparently has a way to measure the ambient light color, and it adjusted the color balance accordingly. As a result, the colors look more correct relative to how your eyes perceived them at the time. Your eyes and brain have rather sophisticated color compenstation built in, so what you perceived was already adjusted to the ambient light color.
This also points out another problem, which is that you are not working with the raw data in post processing. No compenstation is applied to the raw data, so all the pictures would have the same color cast in raw. Cameras often put additional information, like the measured ambient color, into raw files, which some software may use as the default color compensation. If you are using raw, then perhaps you are seeing the different color balance due to the software defaults. In that case you can go and set them all to something of your chosing. One way to do this is to find something in the picture you know should be white, then tell the software to set the color balance so that it comes out white (or some shade of neutral gray). The building at the right would be something to start with, for example.
To prove the points above, here are your second (+2 EV flash) and third (ambient) pictures post-processed to the same color balance and exposure. I used a patch of flat wall on one of the buildings at right as the reference for white and as the common exposure point for both pictures.
Now you can actually see a little effect from the flash in the first picture. This was previously swamped by the dark exposure and the yellow color cast. There are two important things to notice:
- The effect of the flash only extends a small distance into the scene. It is pretty much gone after 10 railroad ties into the picture. Since most of the scene is at much further distance, the flash has no effect on the other objects.
The square law is a powerful effect you can't escape. Even if your flash was 4 times more powerful, its effect would only extend twice as far into the scene, which still wouldn't make any difference for anything but a little further along the railroad track.
- The flash light looks bluish. This is because I compenstated both pictures for the ambient light, which has a yellow cast. Since yellow is corrected to look white, white will look like the inverse of yellow, which is blue.