No need for post processing.
Relative to the camera, the motorcycle and rider are not moving (unless the rider waves the selfie-stick) so they are sharp. The background (clouds and landscape) are changing very little due to the distance, so they also look sharp. The road and nearby shoulder to move quickly relative to the camera, so are subject to motion ...
Tamron manual VC instructions below.
Note in particular:
1. Turn the VC switch OFF when using tripod.
2. The VC mechanism may introduce errors during long exposures.
3. When the shutter button is pressed down halfway, it takes about 1 second for the VC to provide a stable image.
4. When VC is not used, set the switch off.
How to use VC mechanism
1) Set ...
Your results are decidedly magenta and that is the opposite of where they should be. They should be green for an uncorrected bayer RGB.
I suspect that your observed sigmoid correction was heavily influenced by the raw data already having a white balance correction applied in ACR. You may even be getting "doubled correction" at this point, but I don't know.
You are summing the values of each pixel, but your exposureAdjust() function assumes that after conditioning, each pixel will be in the range [0, 1). This is not correct. Assume a pixel's value in each of the 3 input images is, say, 50% full-scale (thus, 213 = 8192). Summing that three times yields 3 * 8192 = 24,576. Then the result after ...